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No. 136 

\S> XV 






United States. Department of the interior. ( U. S. geological survey.) 
Department of the interior | — | Bulletin | of the | United 
States | geological survey | no. 136 | [Seal of the department] | 
Washington | government printing office | 1896 

Second title: United States geological survey | Charles D. 
Walcott, director | — | The | ancient volcanic rocks | of | South 
Mountain, Pennsylvania | by | Florence Bascom | [Vignette] | 

Washington | government printing office I 1896 

8°. 124 pp. 28 pi. 

Bascom (Florence). 

United States geological survey | Charles D. Walcott, di- 
rector | — | The | aucient volcanic rocks | of | South Mountain, 
Pennsylvania | by | Florence Bascom | [Vignette] | 

Washington | government printing office | 1896 

8°. 124 pp. 28 pi. 

[United States. Department of the interior. (V. S. geological survey.) 
Bulletin 136.] J ' 

United States geological survey | Charles D. Walcott, di- 
rector | — | The | ancient volcanic rocks of | South Mountain 
Pennsylvania | by | Florence Bascom | [Vignette] | 

Washington | government printing office | 1896 

8°. 124 pp. 28 pi. 

* !, U ^ T fo D * f TATES ' De P artment °f th * ™te™r. (TT. S. geological survey.) 
Bulletin 136.] y 

Digitized by the Internet Archive 
in 2013 

[Bulletin No. 136.] 

The statute approved March 3, 187D, establishing the United States Geological Survey, contains the 
following provisions : 

"The publications of the Geological Survey shall consist of the annual report of operations, geological 
and economic maps illustrating the resources and classification of the lands, and reports upon general 
and economic geology and paleontology. The annual report of operations of the Geological Survey 
shall accompany the annual report of the Secretary of the Interior. All special memoirs and reports 
of said Survey shall bo issued in uniform quarto series if deemed necessary by the Director, but other- 
wise in ordinary octavos. Three thousand copies of each shall be published for scientific exchanges 
and for sale at the price of publication ; and all literary and cartographic materials received in exchange 
shall be the property of the United States and form a part of the library of the organization ; and the 
money resulting from the sale of such publications shall be covered into the Treasury of the United 

Except in those cases in which an extra number of any special memoir or report has been supplied 
to the Survey by resolution of Congress or has been ordered by the Secretary of the Interior, this 
office has no copies for gratuitous distribution. 


I. First Annual Report of the United States Geological Survey, by Clarence King. 1880. 8°. 79 pp 
1 map.— A preliminary report describing plan of organization and publications. 

II. Second Annual Report of the United States Geological Survey, 1880-'81, by J. W. Powell. 1882 
8°. lv, 588 pp. 62 pi. 1 map. 

III. Third Annual Report of the United States Geological Survey, 1881-'82, by J. W. Powell. 1883 
8°. xviii, 564 pp. 67 pi. and maps. 

' IV. Fourth Annual Report of the United States Geological Survey, 1882-'83, by J. W. Powell. 1884, 
8°. xxxii, 473 pp. 85 id. and maps. 

V. Fifth Annual Report of the United States Geological Survey, 1883-'84, by J. W. Powell. 1885, 
8°. xxxvi, 469 pp. 58 pi. and maps. 

VI. Sixth Annual Report of the United States Geological Survey, 1884-'85, by J. W. Powell. 1885 
8°. xxix, 570 pp. 65 pi. and maps. 

VII. Seventh Annual Report of the United States Geological Survey, 1885-'86, by J. W. Powell. 1888 
8°. xx, 656 pp. 71 pi. and maps. 

VIII. Eighth Annual Report of the United States Geological Survey, 1886-'87, by J. W. Powell. 1889, 
8°. 2 pt. xix, 474, xii pp. 53 pi. and maps ; 1 p. 1., 475-1063 pp. 54-76 pi. and maps. 

IX. Ninth Annual Report of the United States Geological Survey, 1887-88, by J. W. Powell. 1889 
8°. xiii, 717 pp. 88 pi. and maps. 

X. Tenth Annual Report of the United States Geological Survey, 1888-'89, by J. W. Powell. 1890 
8°. 2pt. xv, 774 pp. 98 pi. and maps; viii, 123 pp. 

XI. Eleventh Annual Report of the United States Geological Survey, 1889-'90, by J. W. Powell. 1891 
8°. 2pt. xv, 757 pp. 66 pi. and maps ; ix, 351 pp. 30 pi. 

XII. Twelfth Annual Report of the United States Geological Survey, 1890-'91, by J. W. Powell. 1891 
8°. 2 pt. xiii, 675 pp. 53 pi. and maps; xviii, 576 pp. 146 pi. and maps. 

XIII. Thirteenth Annual Report of the United States Geological Survey, 1891-'92, by J. W. Powell 
1893. 8°. 3pt. vii, 240 pp. 2 maps; x, 372 pp. 105 pi. and maps; xi, 486pp. 77 pi. and maps. 

XIV. Fourteenth Annual Repoi-t of the United States Geological Survey, 1892-'93, by J. W.Powell 
1893. 8°. 2 pt. vi, 321 pp. 1 pi. ; xx, 597 pp. 74 pi. 

XV. Fifteenth Annual Report of the United States Geological Survey, 1893-'94, by J. W. Powell 
1895. 8°. xiv, 755 pp. 48 pi. 

XVI. Sixteenth Annual Report of the United States Geological Survey, 1894-'95, by Charles D 
Walcott. 1895. (Part I, 1896.) 8°. 4 pt. xxii, 910 pp. 117 pi. and maps: xix, 598 pp. 15 pi. and maps 
xv, 646 pp, 23 pi. ; xix, 735 pp. 6 pi. 




I. Lake Bonneville, by Grove Karl Gilbert. 1890. 4°. xx, 438 pp. 51 pi. 1 map. Price $1.50. 

II. Tertiary History of the Grand Canon District, with atlas, by Clarence E. Duttou, Capt. TJ. S. A. 
1882. 4°. xiv, 264 pp. 42 pi. and atlas of 21 sheets folio. Price $10.00. 

III. Geology of the Cornstock Lode and the Washoe District, with atlas, by George F. Becker. 1882. 
4°. xv, 422 pp. 7 pi. and atlas of 21 sheets folio. Price $11.00. 

IV. Comstock Mining and Miners, by Eliot Lord. 1883. 4°. xiv, 451 pp. 3 pi. Price $1.50. 

V. The Copper-Bearing Hocks of Lake Superior, by Roland Duer Irving. 1883. 4°. xvi, 464 pp. 
151. 29 pi. and maps. Price $1.85. 

VI. Contributions to the Knowledge of the Older Mesozoic Flora of Virginia, by William Morris 
Fontaine. 18S3. 4°. xi, 144 pp. 54 1. 54 pi. Price $1.05. 

VII. Silver-Lead Deposits of Eureka, Nevada, by Joseph Story Curtis. 1884. 4°. xiii, 200 pp. 16 
pi. Price $1.20. 

VIII. Paleontology of the Eureka District, by Charles Doolittle Walcott. 1884. 4°. xiii, 298 pp. 
24 1. 24 pi. Price $1.10. 

IX. Brackiopoda and Lamellibranchiata of the Raritau Clays and Greensand Marls of New Jersey, 
by Robert P. Whitfield. 1885. 4°. xx, 338 pp. 35 pi. 1 map. Price $1.15. 

X. Dinocerata. A Monograph of an Extinct Order of Gigantic Mammals, by Othniel Charles Marsh. 
1886. 4°. xvin, 243 pp. 56 1. 56 pi. Price $2.70. 

XI. Geological History of Lake Lahontan, a Quaternary Lake of Northwestern Nevada, by Israel 
Cook Russell. 1885. 4°. xiv, 288 pp. 46 pi. and maps. Price $1.75. 

XII. Geology and Mining Industry of Leadville, Colorado, witli atlas, by Samuel Franklin Emmons. 
1886. 4°. xxix, 770 pp. 45 pi. and atlas of 35 sheets folio. Price $8.40. ' 

XIII. Geology of the Quicksilver Deposits of the Pacific Slope, with atlas, by George F. Becker. 
1888. 4°. xix, 486 pp. 7 pi. and atlas of 14 sheets folio. Price $2.00. 

XIV. Fossil Fishes and Fossil Plants of the Triassic Rocks of New Jersey and the Connecticut Val- 
ley, by John S. Newberry. 1888. 4°. xiv, 152 pp. 26 pi. Price $1.00. 

XV. The Potomac or Younger Mesozoic Flora, by William Morris Fontaine. 1889. 4°. xiv, 377 
pp. 180 pi. Text and plates bound separately. Price $2.50. 

XVI. The Paleozoic Fishes of North America, by John Strong Newberry. 1889. 4°. 340 pp. 53 pi. 
Price $1.00. 

XVII. The Flora of the Dakota Group, a posthumous work, by Leo Lesquereux. Edited by F. H. 
Knowlton. 1891. 4°. 400 pp. 66 pi. Price $1.10. 

XVIII. Gasteropoda and Cephalopoda of the Raritan Clays and Greensand Marls of New Jersey, 
by Robert P. Whitfield. 1891. 4°. 402 pp. 5) pi. Price $1.00. 

XIX. The Penokee Iron Bearing Series of Northern Wisconsin and Michigan, by Roland D. Irving 
and C. R. Van Hise. 1832. 4°. xix, 534 pp. 37 pi. Price $1.70. 

XX. Geology of the Eureka District, Nevada, with atlas, by Arnold Hague. 1892. 4°. xvn, 419 pp, 
8 j)l. Price $5.25. 

XXI. The Tertiary Rhynchophorous Coleoptera of North America, by Samuel Hubbard Scudder. 

1893. 4°. xi, 206 pp. 18 pi. Price 90 cents. 

XXII. A Manual of Topographic Methods, by Henry Gannett, chief topographer. 1893. 4°. xiv, 
300 pp. 18 pi. Price $1.00. 

XXIII. Geology of the Green Mountains in Massachusetts, by Raphael Pumpelly, J. E. Wolff, 
and T. Nelson Dale. 1894. 4°. xiv, 206 pp. 23 pi. Price $1.30. 

XXIV. Mollusca and Crustacea of the Miocene Formations of New Jersey, by Robert Parr Whitfield. 

1894. 4°. 195 pp. 24 pi. Price 90 cents. 
In press : 

XXV. The Glacial Lake Agassiz, by Warren Uphain. 1895. 4°. xxiv, 658 pp. 38 pi. 

XXVI. Flora of the Amboy Clays, by John Strong Newberry; a posthumous work, edited by 
Arthur Hollick. 1895. 4°. 260 pp. 58 pi. 

XXVII. Geology of the Denver Basin. Colorado, by S. F. Emmons, Whitman Cross, and George H. 

In preparation : 

— The Marquette Iron-Bearing District of Michigan, by C. R. Van Hise and W. S. Bayley, with a 
Chapter on the Republic Trough, by H. L. Smyth. 

— The Geology of Franklin, Hampshire, and Hampden counties, Massachusetts, by Benjamin Ken- 
dall Emerson. 

— The Glacial Gravels of Maine and their Associated Deposits, by George H. Stone. 

— . Geology of the Narragansett Basin, by N. S. Shaler. J. B. Woodworth, and August F. Foerste. 

— Fossil Medusa*, by C. D. Walcott. 

— Sauropoda, by O. C. Marsh. 

— Stegosauria, by (). C. Marsh. 

— Brontotherida\ by O. C. Marsh. 

— Report on Silver Cliff and Ten-Mile Mining Districts, Colorado, by S. F. Emmons. 

— Flora of the Laramie and Allied Formations, by Frank Hall Knowlton. 



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4. On Mesozoic Fossils, by Charles A. White. 1884. 8°. 36 pp. 9 pi. Price 5 cents. 

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6. Elevations in the Dominion of Canada, by J. W. Spencer. 1884. 8°. 43 pp. Price 5 cents. 

7. Mapoteca Geologica Americana. A Catalogue of Geological Maps of America (North and South), 
1752-1881, in geographic and chronologic order, by Jules Marcou and John Belknap Marcou. 1884. 
8°. 184 pp. Price 10 cents. 

8. On Secondary Enlargements of Mineral Fragments in Certain Rocks, by R. D. Irving and C. R. 
VanHise. 1884. 8°. 56 pp. 6 pi. Price 10 cents. 

9. A report of work done in the Washington Laboratory during the fiscal year 1883-'84. F. W. Clarke, 
chief chemist. T. M. Chatard, assistant chemist. 1884. 8°. 40 pp. Price 5 cents. 

10. On the Cambrian Faunas of North America. Preliminary studies, by Charles Doolittle Walcott 

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11. On the Quaternary and Recent Mollusca of the Great Basin; with Descriptions of New Forms, 
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12. A Crystallographic Study of the Thinohte of Lake Lahontan, by Edward S. Dana. 1884. 8°. 
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14. The Electrical and Magnetic Properties of the Iron-Carburets, by Carl Barus and Vincent 
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15. On the Mesozoic and Cenozoic Paleontology of California, by Charles x\. White. 1885. 8°. 33 pp. 
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16. On the. Higher Devonian Faunas of Ontario County, New York, by John M. Clarke. 1885. 8°: 
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18. On Marine Eocene, Fresh-water Miocene, and other Fossil Mollusca of Western North America, 
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20. Contributions to the Mineralogy of the Rocky Mountains, by Whitman Cross and W. F. Hille- 
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22. On New Cretaceous Fossils from California, by Charles A. White. 1885. 8°. 25 pp. 5 pi. 
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23. Observations on the Junction between the Eastern Sandstone and the Keweenaw Series on 
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24. List of Marine Mollusca, comprising the Quaternary Fossils and recent forms from American 
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25. The Present Technical Condition of the Steel Industry of the United States, by Phineas Barnes. 
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26. Copper Smelting, by Henry M. Howe. 1885. 8°. 107 pp. Price 10 cents. 

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28. The Gabbros and Associated Hornblende Rocks occurring in the neighborhood of Baltimore, 
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29 On the Fresh-water Invertebrates of the North American Jurassic, by Charles A. White. 1886. 
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30. Second Contribution to the Studies on the Cambrian Faunas of North America, by Charles Doo» 
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31. Systematic Review of our Present Knowledge of Fossil Insects, including Myriapods and Arach- 
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32. Lists and Analyses of the Mineral Springs of the United States (a Preliminary Study), by Albert 
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33. Notes on the Geology of Northern California, by J. S. Diller. 1886. 8°. 23 pp. Price 5 cents. 

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35. Physical Properties of the Iron -Carburets, by Carl Barus and Vincent Strouhal 1886. 8°. 62 
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36. Subsidence of Pine Solid Particles in Liquids, by Carl Barus. 1886 8°. 58 pp Price 10 cents. 

37. Types of the Laramie Flora, by Lester P. Ward. 1887. 8°. 354 pp. 57 pi. Price 25 cents. 

38. Peridotite of Elliott County, Kentucky, by J. S Diller. 1887. 8°. 31 pp. 1 pi. Price 5 cents. 

39. The Upper Beaches and Deltas of the Glacial Lake Agassiz, by Warren Upham. 1887. 8°. 84 
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40. Changes in River Courses in Washington Territory due to Glaciation, by Bailey Willis. 1887. 
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41. On the Fossil Faunas of the Upper Devonian — the Genesee Section, New York, by Henry S. 
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42. Report of work done in the Division of Chemistry and Physics, mainly during the fiscal year 
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43. Tertiary and Cretaceous Strata of the Tuscaloosa, Tombigbee, and Alabama Rivers, by Eugene 
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44. Bibliography of North American Geology for 1886, by Nelson H. Darton. 1887. 8°. 35 pp. 
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45. The Present Condition of Knowledge of the Geology of Texas, by Robert T. Hill. 1887. 8°. 94 
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46. Nature and Origin of Deposits of Phosphate of Lime, by R. A. F. Penrose, jr., with an intro- 
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47. Analyses of Waters of the Yellowstone National Park, with an Account of the Methods of 
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48. On the Form and Position of the Sea Level, by Robert Simpson Woodward. 1888. 8°. 88 pp. 
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49. Latitudes and Longitudes of Certain Points in Missouri, Kansas, and New Mexico, by Robert 
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50. Formulas and Tables to facilitate the Construction and Use of Maps, by Robert Simpson Wood- 
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51. On Invertebrate Fossils from the Pacific Coast, by Charles Abiathar White. 1889. 8°. 102 
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52. Subaerial Decay of Rocks and Origin of the Red Color of Certain Formations, by Israel Cook 
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53. The Geology of Nantucket, by Nathaniel Southgate Shaler. 1889. 8°. 55 pp. 10 pi. Price 10 

54. On the Thermo-Electric Measurement of High Temperatures, by Carl Barus. 1889. 8°. 313 pp. 
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55. Report of work done in the Division of Chemistry and Physics, mainty during the fiscal year 
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56. Fossil Wood and Lignite of the Potomac Formation, by Frank Hall Knowlton. 1889. 8°. 72 
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57. A Geological Reconnaissance in Southwestern Kansas, by Robert Hay. 1890. 8°. 49 pp. 2 pi. 
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58. The Glacial Boundary in Western Pennsylvania, Ohio, Kentucky, Indiana, and Illinois, by George 
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59. The Gabbros and Associated Rocks in Delaware, by Frederick D. Chester. 1890. 8°. 45 pp. 
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60. Report of work done in the Division of Chemistry and Physics, mainly during the fiscal year 
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61. Contributions to the Mineralogy of the Pacific Coast, by William Harlowe Melville and Waldemar 
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62. The Greenstone Schist Areas of the Menominee and Marquette Regions of Michigan, a contri- 
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63. A Bibliography of Paleozoic Crustacea from 1698 to 1889, including a list of North American 
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64. A report of work done in the Division of Chemistry and Physics, mainly during the fiscal year 
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65. Stratigraphy of the Bituminous Coal Field of Pennsylvania, Ohio, and West Virginia, by Israel 
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67. The Relations of the Traps of the Newark System in the New Jersey Region, by Nelson Horatio 
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68. Earthquakes in California in 1889, by James Edward Keeler. 1890. 8°. 25 pp. Price 5 -cents. 

69. A Classed and Annotated Bibliography of Fossil Insects, by Samuel Hubbard Scudder. 1890. 
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70. Report on Astronomical Work of 1889 and 1890, by Robert Simpson Woodward 1890. 8°. 79 pp. 
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71. Index to the Known Fossil Insects of the World, including Myriapods and A rachnids, by Samuel 
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72. Altitudes between Lake Superior and the Rocky Mountains, by Warren Uphani. 1891. 8°. 
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73. The Viscosity of Solids, by Carl Barus. 1891. 8°. xii, 139 pp. 6 pi. Price 15 cents. 

74. The Minerals of North Carolina, by Frederick Augustus Genth. 1891. 8°. 119 pp. Price 15 

75. Record of North American Geology for 1887 to 1889, inclusive, by Nelson Horatio Darton. 1891. 
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6. A Dictionary of Altitudes iu the United States (second edition), compiled by Henry Gannett, 
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77. The Texan Permian and its Mesozoic Types of Fossils, by Charles A. White. 1891. 8°. 51 pp. 
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8. A report of work done in 'the Division of Chemistry and Physics, mainly during the fiscal year 
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}. A Late Volcanic Eruption in Northern California and its Peculiar Lava, by J. S. Diller. 1891. 8°. 
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). Correlation papers— Devonian and Carboniferous, by Henry Shaler Williams. 1891. 8°. 279 pp. 
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81. Correlation papers— Cambrian, by Charles Doolittle Walcott. 1891. 8°. 447 pp. 3 pi. Price 
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82. Correlation papers— Cretaceous, by Charles A. White. 1891. 8°. 273 pp. 3 pi. Price 20 cents. 

83. Correlation papers— Eocene, by William Bullock Clark. 1891. 8°. 173 pp. 2 pi. Price 15 cents. 
14. Correlation papers— Neocene, by W. H. Dall and G. D. Harris. 1892. 8°. 349 pp. 3 pi. Price 

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85. Correlation papers — The Newark System, by Israel Cook Russell. 1892. 8°. 344 pp. 13 pi. 
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90. A report of work done in the Division of Chemistry and Physics, mainly during the fiscal year 
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91. Record of North American Geology for 1890, by Nelson Horatio Darton. 1891. 8°. 88 pp. Price 
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92. The Compressibility of Liquids, by Carl Barus. 1892. 8°. 96 pp. 29 pi. Price 10 cents. 

93. Some Insects of Special Interest from Florissant, Colorado, and other points in the Tertiaries of 
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95. Earthquakes in California in 1890 and 1891, by Edward Singleton Holden. 1892. 8°. 31 pp, 
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96. The Volume Thermodynamics of Liquids, by Carl Barus. 1892. 8°. 100 pp. Price 10 cents. 

97. The Mesozoic Echinodermata of the United States, by William Bullock Clark. 1893. 8°. 20? 
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98. Flora of the Outlying Carboniferous Basins of Southwestern Missouri, by David White. 1893 
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99. Record of North American Geology for 1891, by Nelson Horatio Darton. 1892. 8°. 73-pp 
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100. Bibliography and Index of the Publications of the U. S. Geological Survey, 1879-1892, by Philip 
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101. Insect Fauna of the Rhode Island Coal Field, by Samuel Hubbard Scudder. 1893. 8°. 27 pp 
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102. A Catalogue and Bibliography of North American Mesozoic Iuvertebrata, by Cornelius Breck 
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103. High Temperature Work in Igneous Fusion and Ebullition, chiefly in relation to pressure, b), 
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104. Glaciation of the Yellowstone Valley north of the Park, by Walter Harvey Weed. 1893. 8 a 
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105. The Laramie and the overlying Livingston Formation in Montana, by Walter Harvey Wee<\. 
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107. The Trap Dikes of the Lake Champlain Region, by James Furman Kemp and Vernon Free 
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108. A Geological Reconnoissance in Central Washington, by Israel Cook Uussell. 1893. 8°. K>8pp 
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109. The Eruptive and Sedimentary Rocks on Pigeon Point, Minnesota, and their contact phenora 
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111. Geology of the P.ig Stone Gap Coal Field of Virginia and Kentucky, by Marius P.. Campbell 
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113. A report of work done in the Division of Chemistry during the fiscal years 1891-'92 anj 
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114. Earthquakes in California in 1893, by Charles D. Perrine. 1894. 8°. 23 pp. Price 5 cents. 

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No. 136 




189 6 









1 8 9 6 



Letter of transmittal ■ « . 11 

Introduction 13 

Chapter I. An account of geological surveys in the South Mountain below 

the Susquehanna 14 

Historical review 11 

Bibliograp 1, 19 

Chapter II. Geological relationship of the rocks of the Monterey district- ... 20 

General description of the area studied 20 

Extent and character of the three rock types 20 

Sedimentary rocks 21 

Areal distribution 21 

Structural features 21 

Thickness 21 

Age and superposition 21 

Acid eruptives 23 

Areal distribution 23 

Character 23 

Previous descriptions 23 

Economic value 24 

Basic eruptives 24 

Areal distribution 24 

Character 24 

Previous descriptions 25 

Ore deposits of the Monterey district 25 

Comparative age of sedimentary and igneous rocks 27 

Contacts described 27 

Conclusions 28 

Comparative age of acid and basic eruptives 29 

Field relations 29 

Probable explanation 29 

Summary 30 

Chapter III. Petrographical description of the Cambrian rocks 31 

Macroscopical description 31 

Microscopical description 31 

Quartzite 31 

Slates 33 

Chemical analysis „ 33 

Summary 34 

Chapter IV. Petrographical description of the acid eruptives 35 

Nomenclature 35 

Quartz-porphyries 39 

Distribution 39 

Macroscopical description 39 

Microscopical description 39 

Phenocrysts 39 

Feldspar 39 

Quartz 40 



Chapter IV — Continued. 

Quartz-porphyries— Continued. 

Microscopical description— Continued. 

Groundmass .-. 

. 41 

Accessory constituents * i 

Aporhyolites . 2 

Distribution , 9 

Macroscopical description 42 

Microscopical description 44 

Phenocrysts ,. 

Feldspar and quartz 44 

Other porphyritic constituents 45 

Groundmass 4g 

Fluidal structure 4g 

Micropoikilitic structure 47 

Spherulitic structure 51 

Chain spherulites 53 

Axiohtic structure 54 

Ehyolitic structure 54 

Lithophysal structure 55 

Micropegmatitic structure : 55 

Perlitic structure ... zx 

~ — — 00 

Amygdaloidal structure 55 

Taxitic structure 57 

Summary of proof of devitrification 57 

Opinions of petrographers , 59 

Chemical composition of the acid eruptives 61 

Acid volcanic breccia r o 

Distribution po 

™f "."""'.."::."".".".::.".:::"::".:::'.:::::::: 63 

r low breccias po 

Tuffaceous breccias n> 

Metamorphosed acid eruptives : sericite-schists and slates 64 

Summary fifi 

Chapter V. Petrographical description of the basic eruptives C8 

Nomenclature CQ 


Melaphyres and augite-porphyrites 69 

Distribution pq 

Macroscopical description 79 

Microscopical description 72 

Original structures 72 

Secondary structures 73 

Original constituents 73 

Secondary constituents 74 

Accessory minerals 77 

Discussion of chemical analyses 7g 

Basic slates 7 o 

Distribution and description 79 

Basic pyroclastics 80 

Distribution and description gg 

Crushed porphyrites gO 

Tuffaceous breccia gQ 

Ash «"™..™"™! 80 

Summary g.. 



Chapter VI. Summary of conclusions 82 

Evidence of the eruptive character of the two rock types 82 

Field evidence 82 

Schistosity 82 

Lamination 82 

The slates s 82 

Absence of gradation between igneous and clastic rocks 83 

Surface-flow features 83 

Petrographical evidence ^ 83 

Structural ■. 83 

Mineralogical -. 84 

Chemical 84 

Original rock types 84 

Acid igneor" - ocks 84 

Basic igneous rocks „ ■. 85 

Similar rocks in other regions 8(5 

Literature 87 



Plate I. Map showing location of Monterey district 13 

II. Panoramic view of mountains of the Monterey district 16 

III. Geologic and topographic map of the Monterey district 20 

IV. Crumpling of sandstone at the east end of the tunnel through Jacks 

Mountain on the Gettysburg Railroad 22 

V. Sections through the Monterey district 24 

VI. Junction of felsite and sandstone on the old Tapeworm Railroad 28 

VII. Flow structure in aporhyolite, Monterey district 42 

VIII. Flow structure in aporhyolite. Raccoon Creek 44 

IX. Spherulitic aporhyolite, Monterey district 46 

X. Aporhyolite with spherulites in layers 48 

XI. Lithophysa>, Raccoon Creek 54 

XII. Flow hreccia 62 

XIII. Acid hreccia, Raccoon Creek 64 

XIV. Sericite-schist, Gettyshurg Railroad 66 

XV. Thin sections : a, quartzite ; b, feldspar crystal 96 

XVI. Thin sections: a, perthitic structure in feldspar; &, stretched feld- 
spar crystal 98 

XVII. Thin sections: a, quartz-albite mosaic filling crack in feldspar crys- 
tal ; b, micropoikilitic structure 100 

XVIII. Thin sections : a, flow structure in an aporhyolite - x b, chain spherulites 

in an aporhyolite 102 

XIX. Thin sections: a, b, augite-porphyrite in ordinary and in polarized 

light 104 

XX. Thiu sections : a, b, perlitic parting in an aporhyolite in ordinary 

and in polarized light 106 

XXI. Thin sections: a, perlitic parting in an aporhyolite; b, axiolites in 

an aporhyolite 108 

XXII. Thin sections : a, b, altered and unaltered spherulites in aporhyolites- 110 

XXIII. Thin sections: a, altered spherulites in ordinary and in polarized 

light ; b, chain spherulites with phenocryst 112 

XXIV. Thin sections : a, b, rhyolitic structures in aporhyolites 114 

XXV. Thin sections : a, rhyolitic structure in an aporhyolite ; b, piedmont- 

ite 116 

XXVI. Thin sections : a, b, amygdaloidal aporhyolites 118 

XXVII. Thin sections: a, b, tridymite spherulites in an aporhyolite 120 

XXVIII. Thin sections : a, augite-porphyrite ; b, melaphyre 122 



Department of the Interior, 
United States Geological Survey, 

Baltimore, Md., March 17, 1894. 
Sir: I have the honor of transmitting herewith, for publication as a 
bulletin of the Survey, a paper by Miss Florence Bascom on the 
Ancient Volcanic Eocks of South Mountain, Pennsylvania. 

This work was done in the field during- the summer of 1892 and in 
my laboratory during the winter of 1892-93. That part of it relating 
to the acid eruptive rocks was accepted in June, 1893, as a thesis for 
the degree of doctor of philosophy at the Johns Hopkins University. 

Since that time the paper has been amplified and improved, and I 
believe that the detailed investigations which it describes will prove to 
be of permanent value in elucidating the character and history of 
ancient volcanic rocks, not merely in South Mountain, but in many 
other regions of the Eastern United States. 
Very respectfully, 

George H. Williams, 

Hon. J. W. Powell, 

Director United States Geological Surveij. 



By Florence Bascom. 


The mountain range known in Vermont as the Green Mountains, in 
Massachusetts as the Taconic Mountains, and in New York and New 
Jersey as the Highlands, is the South Mountain in Pennsylvania and 
Maryland, the Blue Eidge in Virginia, and the Smoky Mountains in 
North Carolina. The South Mountain in Pennsylvania lies just east 
of the middle of the State, and stretches from Maryland north and 
east in a sickle-shaped curve toward the Susquehanna, While in the 
! New England States the mountains of this range rise to heights from 
3,000 to 4,000 feet above sea level, and in its southern extension from 
4,000 to 7,000 feet, in Pennsylvania its summits rarely exceed 1,500 feet. 

South Mountain is 50 miles in length, and 10 miles wide at its broad- 
i est point. It covers from 150 to 175 square miles, and abundantly 
exposes distinct and interesting rock types. These rocks, prevailing 
throughout the entire South Mountain range, have long been known 
to geologists, although their true character was not recognized until 

In December, 1892, as the result of field work on the part of Dr. G. 
I H. Williams in the northern and of the writer in the southern portion 
I of South Mountain, there appeared a preliminary description 1 of two of 
the rock types, in which their identification as ancient volcanics was 
announced. In this bulletin it is proposed to substantiate that identifi- 
cation with more detailed proof. It is further proposed to show that 
these ancient igneous rocks were, at the time of their consolidation, 
identical in character with their recent volcanic analogues, and that 
their present differences are due to subsequent changes, chief among 
which has been devitrification. It is also proposed to recognize these 
facts in the nomenclature. 

The petrographical features of the third and only remaining rock 
type of the South Mountain will also be described in some detail. 

A brief report upon the previous geological work accomplished in 
the South Mountain, and some account of the structural features of 
the mountain and the age of its rocks, will precede the petrographical 

'The volcanic rocks of the South Mountain in Pennsylvania and Maryland: Am. Jour. Sci., 3d 
series, Vol. XLIV, Dec, 1892, pp. 482-496, PI. I. lleprinted in the Scientific American for Jan. 14,1893. 





The first topographical description of South Mountain appeared as 
early as 1755. 1 It was made by Lewis Evans, of Philadelphia, who 
describes the South Mountain with a fair degree of accuracy, as "not 
in ridges like the Endless Mountains, but in small, broken, steep, stony 
hills; nor does it run with so much regularity." He continues: "In 
some places it gradually degenerates to nothing, not to appear again 
for some miles, and in others it spreads several miles in breadth." 

In a publication 2 which appeared in Germany in 1787 several pages 
are devoted to a general description of South Mountain. Two of the 
type rocks (the sandstone and the porphyry) were noted and aptly 
described, as the following quotation shows : 

One finds here and there gray laminated sandstones with quartz veins; fragments 
of coarse ferruginous quartz. At one spot on the road [from Sharpsburg to Freder- 
ick town] I found hlocks of gray -reddish porphyry with little transparent quartz 
grains intermixed, and milk-white opaque feldspars. *■ * * The South Mountain 
in its entire extent contains rich crevices, gangues, and nests of ore, especially of 
iron and copper. * * * I have still to add, from the observations made upon 
this journey, that the eastern slope is gentler and more gradual than the western. 

The most important publications on the South Mountain have 
appeared under the auspices of the various surveys of Pennsylvania 
and Maryland. The First Geological Survey of Pennsylvania was 
organized in 1836 under the distinguished geologist, Henry D. Rogers. 

The difficulties encountered, however, were so great that it was not 
until 1858 that the two quarto volumes of the survey were issued. 
Professor Rogers deals somewhat cursorily with the South Mountain 
region. He says : 3 

In its geological constitution, this tract is without much variety, for it contains 
scarcely any rocks except those of the Primal series. It is doubtful if the true 
gneissic rocks anywhere reach the surface withm its borders, and only in one or two 
localities have even the lowest members of the Auroral limestones been met with 
covering the upper Primal slates. Even of intrusive igneous rocks, it embraces a 
singularly small amount, those met with being chiefly greenstones and trap rocks. 

'Analysis of a Map of the British Colonies in America, 1755. 

5 Schopf's Beytrage zur mineralogischen Kenntniss ties ostlichen Theils von Nordamerika und 
seine Gebiirge, chapter 30, pp. 96-101, Erlangen, 1787. A translation of this work has been made by 
Prof. John M. Clark, of the New York State Museum of Natural History, to whose kindness the 
writer its indebted for a copy of the unpublished manuscript of the pages relating to South Mountain. 

3 Geology of Pennsylvania, Vol. I, Part II, pp. 203-209, 1858. 


The geological structure or mode of stratification of this belt is equally simple. 
The whole tract consists of two or three groups of high, narrow, nearly parallel 
anticlinal ridges, expanding and subdividing toward the southwest. These are 
composed of the Primal white sandstone. Between them are high parallel valleys 
and plateaus of the Primal upper slates, which, from being softer and more fissile, 
have been worn and trenched by the plowing force of waters to somewhat lower 
levels than the more resisting, . ter cemented sandstones. The crests of the ridges 
are therefore stony and rugged, their flanks usually smoother, being formed chiefly 
of the slate. 1 

That Professor Rogers did not include in the " singularly small 
amount" of igneous rocks the rocks forming the valleys, the mountain 
flanks, and even the summits of the mountains, is further indicated by 
the following: 

Another section across the mountain more to the southwest extends from south to 
north along the Baltimore and Carlyle turnpike. The first important stratum of the 
hills is the usual gray siliceous altered rock, so common along their southern side. 

North of this, about 3 miles from Petersburg, occurs the dark green slate, with its 
epidote and white intrusive quartz. Succeeding this is an extremely compact sili- 
ceous altered slate, and beyond this a reddish gray rock, of the same series, containing 
specks of reddish feldspar and small veins of epidote, and near this the fissile tal- 
cose rock, several times mentioned before. * The summit of the ridge exhibits 

a dark blue and greenish blue indurated rock, weathering a dark brown, and evi- 
dently very ferruginous. It appears to be a band of the Primal slate in a highly met- 
amorphic condition, approaching jasper. * * * These lower Primal slates are 
highly indurated, and even decidedly crystalline, containing in some of their layers 
segregated specks, and even half-formed geodes of epidote and other minerals. 

On page 206 of the same report Professor Eogers has figured a sec- 
tion across South Mountain, along the Gettysburg Railroad, from Fair- 
field to Monterey Springs. This section shows stratified rocks lying 
in a series of anticlinal flexures, which accord rather with Professor 
Rogers's conception of "rock waves" than with his observed dips. 
These dips are, with a single exception, to the southeast. Accompany- 
ing the section is the following description, which bears directly upon 
the portion of South Mountain that is particularly discussed in this 
bulletin : 

Passing now to the eastward of the Green Ridge axis we cross a high slope of 
slate, apparently the upper Primal, in a synclinal fold, and then traverse a succes- 
sion of outcrops of the Primal white sandstones and slates to the eastern base of the 
high land called Jack's Mountain, at the foot of which the older rocks disappear 
under the Mesozoic red sandstone of the plain of Adams County. 

The exposures in the sandstone near the tunnel opposite Jack's Mountain indicate 
a probable thickness of 1.000 feet. Near the tunnel at the northwest side of the 
mountain there is a hard epidotic rock, and not far from it a highly altered greenish 
slate, a rock found in several other localities farther west and containing layers of 
gray slate, spotted with epidote. Farther west occurs epidote with asbestus. Near 
Mmie Branch search was made many years ago for copper ore, but nothing was found 
to justify the expectation of finding a productive vein of that mineral. A small 
quantity of copper ore was once obtained and a furnace built for smelting it in a 
small ridge north of Jack's Mountain, but the exploration was abandoned. The 
metal occurs in the form of a green and blue carbonate, with a little native copper. 
Evidently the ore is not abundant. 

Tlio italics in this and tli«' following quotations are tho writer's. 


Many of the characteristic features of the South Mountain rocks were 
aptly described by Professor Rogers, yet, plainly, their nature was not 
fully understood nor their importance appreciated at the close of the 
First Geological Survey of Pennsylvania. In 18G0 the Primal series 
of Pennsylvania, as it occurs in Maryland, was thus characterized by 
Tyson: 1 

1. A hard sandstone, made up of grains of quartz, with occasionally grains of feld- 
spar and kaolin. 

2. A slate, varying in color from gray to brownish and greenish. It is ranked as 
an argillite, but portions of it assume a marked talcose appearance, especially in the 
Catoctin Mountain, and in parts of Middletown Valley, where it has been much dis- 
turbed and altered by proximity to intrusive rocks. These last consist of amphibo- 
lites (trap), porphyries, amygdaloid, serpentine, and epidote. This last-named rock 
is extensively developed, both in large masses and intercalated between the slates. 

The Second Geological Survey of Pennsylvania was organized in 1874 
under Prof. J. P. Lesley, who had been an assistant to Professor Rogers 
in the earlier survey. 

With improved facilities for scientific work, and with more accurate 
methods of mapping, a new survey of South Mountain was undertaken 
by Dr. Persifor Frazer, with A. E. Lehman as topographical assistant. 
Five sections (Nos. 7, 8, 9, 10, 11), more or less incomplete, were made 
through the mountain, and as the result of his investigations Dr. Frazer 
says : 2 

It is apparent that the great South Mountain is composed essentially of two 
groups of rocks, the lower (and along this line, the northwestern) consisting of 
various modifications of the quartz conglomerate above spoken of, and in which 
quartz occurs in various forms. 

The upper and southeasterly group is felsitic in character, but contains, also, large 
beds of hydromica and chlorite schists intersected by veins of milk quartz, while 
the orthofelsite presents every variety of appearance, from a sandy and earthy slate, 
in which the crystals of orthoclase are very much decomposed, indeed, are some- 
times almost clay, through the jasper-like variety to the massive and coarsely por- 
phyritic structure in which it is suited to be used as an ornamental building stone. 

In the same year (1877) Dr. Frazer published a further account 3 of 
the nature and origin of these "orthofelsites." This account was in 
substance twice repeated by him, in 1879 4 and in 1880 : 5 

The rocks of this region [South Mountain] may be divided into two great series — 
a western (underlying), of which the characteristic strata are composed of quartzite 
and of arenaceous schist containing quartz pebbles (Mountain Creek rock), and the 
eastern (overlying) of hydromica and chlorite schists, and orthofelsite, both porphy- 
ritic and unporphyritic. Both these series show indications of having been pene- 
trated by dikes of plutonic character within this area. 

The porphyry, which carries the copper in this region, shows no character of igneous 
action, hut occurs in coarse and thin beds, more or less disintegrated, and in certain 

'Report on the Geology of Maryland, Jan., 1860, pp. 34, 35. 

2 Report of Progress in the Counties of York, Adams, Cumberland, and Franklin, 1877, CC, pp 

3 The copper ores of Pennsylvania: Polytechnic Review, Nos. 16 and 17, Vol. Ill, April, 1877. 
-»Trans. Am. Inst. Min. Eng., Vol. VII, 1879, p. 338. 
6 Second Geol. Survey, Pa., CCC, Appendix, 1880, pp. 312-313. 



5 ;i'iS':P -»* 






localities reduced, almost to the state of kaolin. Nothing which might correspond 
to the term sandstone was ohserved, though all the above sediments were free of 
grit and sandy particles. * * * It seems fair to conclude that the region of the 
copper-bearing rocks belongs to the Huronian cycle. 

With these views Dr. T. Sterry Hunt expressed entire concurrence. 1 
Dr. Hunt had previously made some study of the South Mountain 

rocks, and published, at various times, his observations concerning 

them. In 1876 he said: 2 

In the southern part of Pennsylvania, to the west of Gettysburg, this moun- 
tainous belt, rising between the Mesozoic on the east and the great limestone valley 
on the west, presents an immense development of a peculiar type of crystalline 
rocks which I detected there last year, and which has a considerable geological 
importance. It is a bedded petrosilex, grayish, reddish, or purplish in color, some- 
times granular but more often jasper-like in texture, and frequently porphyritic from 
the presence of small crystals of orthoclase-feldspar or of glassy quartz. There is 
found here a great breadth of this rock distinctly bedded, presenting different varie- 
ties, and alternating with dioritic, or diabasic, epidotic, and chloritic rocks, with 
argillites, in which are sometimes included thin beds of the petrosilex, the strata 
generally dipping at high angles to the east. 

These rocks Dr. Hunt provisionally referred to a position near the 
base of the Huronian division, adding: 

This petrosilex is identical in its lithological character with the hiilleilinta, or 
stratified flint rock of the Swedish geologists, which is by them assigned a similar 
position, i. e., above the most ancient gneisses. 

In 1878 Dr. Hunt expressed essentially the same views, 3 and in 1879 4 
he opposed the correlation of the South Mountain rocks with the copper- 
bearing rocks of Lake Superior (Keweenawan series), although at an 
earlier date he notes a resemblance. He says : 5 

I may also note that I have observed bedded petrosilex rocks like those just 
noticed [South Mountain porphyries] to the north of Lake Superior, both in an 
island south of St. Ignace and on the adjacent mainland. The conglomerates or 
breccias, which, in the rocks of the Keweenawan series on the south shore of the 
lake, include the native copper of the Calumet and Hecla and the Boston and Albany 
mines, are also made up of the ruins of a precisely similar petrosilex porphyry. 

In October, 1879, Mr. J. F. Blandy made a brief reconnoissance of 
the lower portion of the South Mountain with not unfruitful results. 
He makes a suggestive correlation of the copper-bearing rocks of 
southern Pennsylvania with the Lake Superior copper formation, 6 
thus recognizing the volcanic nature of the greenstones, at least, 
which he called "amygdaloidal trap." The acid rocks still remain 

In the final report of the Pennsylvania survey, 7 Professor Lesley 

' Trans. Am. Inst. Min. Eng., Vol. VII, 1879, p. 339. 
2 Proc. Am. Ass. Adv. Sci., 1876, pp. 211-212. 
i Second Geol. Surv. Pa., Vol. E, p. 193. 

4 Trans. Am. Inst. Min. Eng., Vol. VII, p. 331. 

5 Proc. Am. Ass. Adv. Sci., 1876, p. 211. 

« Trans Am. Inst. Min. Eng., Vol. VII, pp. 331-333. 

7 Final Peport of the Pa. Geol. Surv., Lesley, Summary. Vol. 1, 189?, 

Pull, 136 ^ 


gives m substance the views of Dr. Frazer, which have already been 
quoted. He refers to Dr. Frazer's " section 8" as representative of 
the South Mountain rocks and structure. In this section , as in the 
others in which the orthofelsite appears, notably section 9, the bedded 
orthofelsite is represented as overlying the quartzose conglomerate. 
Professor Frazer concludes his summary with the statement that "it 
is hard to avoid the inference that our South Mountain rocks repre- 
sent the Iluronian section of Murray and Logan." 

The geological map of Adams, Cumberland, and Franklin counties 
made by this final Pennsylvania survey refers all the rocks of the 
South Mountain region by the use of a single color to the "Azoic 

The Second Geological Survey of Pennsylvania, while recognizing 
the extent and crystalline character of the South Mountain rocks and 
emphasizing the absence of the Primal of Rogers, still failed to solve 
the problems of the structure of the region and the age and origin of 
its rocks. 

Professor Lesley has been very ready to acknowledge the incomplete- 
ness of the survey in the South Mountain, and since Mr. Walcott's 
determination of the age of its sedimentary rocks has appeared, he has 
expressed himself as desiring a new investigation of that region. 
While he claims that the survey of Pennsylvania "has been so minute 
and complete that comparatively little remains to be desired in the 
future," he adds: "The geology of the South Mountain is therefore 
[because of the new aspect given to it by the investigation of Mr. 
Walcott] in a very unsatisfactory condition and requires, in fact, a 
special and protracted investigation in the field. It is the most unsat- 
isfactory part of the work of the Geological Survey of the State." 1 

Subsequent workers in the South Mountain are indebted to the 
Survey for the superior topographical maps of the mountain made on 
a scale of 1 inch to 1,600 feet (about 3 inches to the mile) by A. E. 
Lehman. These maps are invaluable to the field geologist and render 
possible accurate areal mapping. Owing to a confusion of cleavage 
planes and bedding the dips recorded on these maps are not reliable. 

While the geological explorations of the South Mountain have been 
careful and minute and conducted by able geologists, the petrography 
of its rocks has never been thoroughly investigated. The microscope 
has not been used to assist in determining the nature and origin of the 
rocks, and to correct impressions colored by preconceived ideas or by 
an experience more or less limited to sedimentary structures. Under 
microscopic scrutiny and the comparative study of recent lavas, an 
increasing number of the so-called sedimentary rocks are proving to 
be igneous in origin. That such has proved to be the case in the South 
Mountain, while not surprising, is a fact of considerable significance, 
both because of characteristics which the rocks themselves possess and 

1 Report of the Board of Coram., pp. 5-6, 1893. 


because they form an important part of a belt of similar rocks extend- 
ing northward and southward along the Atlantic Coast. 

Following is a list of publications on the rocks of South Mountain, 
Pennsylvania : 


1755. Lewis Evans. Analysis of a map of the British colonies. 

1787. Schopf. Beytriige zur mineralogischeii Kentniss des ostlichen Theils von Nord- 

amerika unci seiner Gehiirge, chap. 30, pp. 66-101. 
1858. Rogers. Geology of Pennsylvania, Vol. I, pp. 203-209. 
1860. Tyson. Report on the Geology of Maryland, pp. 31-35. 
1875-1877. Frazer. Second Geological Survey of Pennsylvania; Report of progress 

in the counties of York, Adams, and Franklin, Vol. CC, p. 285. 

1876. Hunt. Proc. Am. Ass. Adv. Sci., pp. 211-212. 

1877. Frazer. Copper ores of Pennsylvania; Polytechnic Review, Vol. Ill, p. 170. 

1878. Hunt. Second Geological Survey of Pennsylvania, Vol. E, p. 193. 

1879. Frazer and Hunt. Trans. Am. Inst. Min. Eng., Vol. VII, pp, 331, 332, 338. 

1879. Blandy. The Lake Superior copper rocks in Pennsylvania; Trans. Am. Inst. 

Min. Eng., Vol. VII, p. 331. 

1880. Frazer. Second Geological Survey of Pennsylvania, Vol. CCC. Appendix. 
1883. Frazer. Iron ores of the middle James River; Trans. Am. Jour. Min. Eng., 

Vol. II, p. 203. 
1883-1884. Frazer. An hypothesis of the structure of the copper helt of South 

Mountain ; Trans. Am. Inst. Min. Eng., Vol. XII, p. 82. 
1883-1881. Henderson. The copper deposits of South Mountain; Trans. Am. Inst. 

Min. Eng., Vol. XII, p. 90. 

1891. H. R. Geiger and A. Keith. The structure of the Blue Ridge near Harpers 

Ferry ; Bull. Geol. Soc. Am., Vol. II, pp. 155-161. 

1892. Lesley. A Summary Description of the Geology of Pennsylvania, Vol. I, p. 116. 
1892. G. H. Williams. The volcanic rocks of South Mountain, in Pennsylvania 

and Maryland; Am. Jour. Sci., Vol. XLIV, pp. 182-496, PI. X. Reprinted 
in the Scientific American, Jan. 14, 1893. 
1892. C. D. Walcott. Notes on the Camhrian rocks of Pennsylvania and Maryland, 
from the Susquehanna to the Potomac; Am. Jour. Sci., Vol. XLIV, pp. 

1892. A. Keith. The geologic structure of the Blue Ridge in Maryland and Vir- 

ginia; The American Geologist, Vol. X, No. 6, pp. 362-369. 

1893. G. H. Williams. Piedmontite and scheelite from the ancient rhyolite of 

South Mountain, Pennsylvania; Am. Jour. Sci., Vol. XLVI, pp. 50-57. 
1896. C. 1). Walcott. The Cambrian rocks of Pennsylvania; Bull. U. S. Geol. Sur- 
vey No. 134. 





This bulletin will treat chiefly of the geology of that portion of South 
Mountain which lies in Franklin and Adams counties and is embraced 
between the lower Cambrian quartzites of the Green Ridge on the 
west and the Triassic red sandstones of the plains on the east. The 
southern limit is the State line, while a parallel line passing through 
the Russel copper mine, some 6 miles to the north, furnishes a northern 
boundary. 1 (See PL I.) Well- wooded mountains (Green Ridge, Mou 
terey Peak, Pine Mountain, Jacks Mountain, Raven Rock Mountain, 
Haycock Mountain, etc.), varying in altitude from 1,500 or 1,850 feet, 
with a general trend from northeast to southwest, inclosing high 
plateau-like valleys and sloping toward the eastern plains, constitute 
a region of exceptional natural beauty and geological interest. (See 
PI. II.) 

Though but a limited portion of an extended South Mountain area, 
this district is in many respects quite typical and furnishes ample mate- 
rial for some general conclusions in regard to the geology of the entire 
area. Although, because of the short time at the disposal of the writer, 
detailed field work was limited to the Monterey district, the petro- 
graphical study included material collected throughout the South 


Three distinct rock types are readily recognized. (PI. III.) 

1. A siliceous sedimentary rock, represented by aquartzose conglom- 
erate, a sandstone, and a quartzite. This is rarely accompanied by an 
interbedded argillaceous slate. 

2. An acid volcanic rock, which shows all phases of crystallization, 
from a spherulitic rhyolite to a true quartz -porixhyry, is amygdaloidal 
or compact, is accompanied by pyroclastics and breccias, and is some- 
times sheared into a perfectly fissile slate or sericite- schist. 

3. A basic, noncrystalline, volcanic rock, which is usually amygda- 
loidal, massive, or more frequently schistose, and is also accompanied 
by j)yroclastics and breccias and sheared to a slate. 

1 As a matter of convenience this area will be calle4 the Monterey district, 




The slates of the region thus prove to be both sedimentary and 
igneous. The former are argillaceous. The latter are either acid or 
basic, and are far more abundant than the former. 


Area! distribution. — The first-mentioned of the rock types occupies 
the high altitudes only, Green Ridge, Monterey Peak, Pine Mountain, 
and Jacks Mountain being largely formed of it, while it caps Haycock 
Mountain and the foothill to the east of Jacks Mountain. 

Structural features. — The strike of the sediments is uniformly north- 
west and southeast, with a mean dip of about 45 degrees southeast. 
There are some exceptious to this southeasterly dip, notably at Monterey 
Peak, where the dip is northwest, forming, with the Green Ridge sand- 
stone, a gentle syncline. (See PI. Y.) 

In the Jacks Mountain quartzite the bedding is exceedingly obscure 
and the cleavage very marked. There is an opportunity for error in 
determining the dip by the confusion of the two. 

At the northeast end of the tunnel which is cut through a spur of 
Jacks Mountain, on the Gettysburg Railroad, stratification is dis- 
tinctly visible, and shows that minor crumpling of the sediments as 
well as folding on a large scale resulted from their uplift. The rock 
is here folded in a series of small anticlines and synclines. (See PI. IY.) 

Thickness. — Professor Rogers estimated the thickness of the sand- 
stone at 1,000 feet. Professor Lesley, on the other hand, considers it 
"immensely thick,'' and states that Frazer's " section 11" shows 32,000 
feet of quartzite and 61,000 feet of schistose conglomerate. 1 

Walcott 2 and Keith, 3 in their examination of a section of the same 
quartzite, sandstone, and conglomerate, displayed to the west of the 
Monterey district, agree essentially with Professor Rogers's estimate 
(1,000-1,200 feet). The writer did not find sufficient data within the 
Monterey district for a reasonably correct estimate of the thickness of 
the quartzite. There are undoubtedly a very great number of minor 
crumplings and foldings resulting in compressed synclines, anticlines, 
and thrust planes in Jacks Mountain. Since there are no means of 
determining the number of these folds, no estimate can be made of the 
thickness of the formation. If the minor folds are ignored, the dip and 
horizontal extent of the sediments indicate a thickness which is enor- 
mous, an indication unsustained by sections only 2 miles to the west. 
Probably the estimate made by Mr. Walcott (1,200 feet) in that more 
favorable locality will be approximately correct for the Jacks Mountain 

Age and superposition. — These siliceous elastics are the " Primal white 

1 Summary, Final Report Geol. Pa., Vol. 1, 1892, pp. 145-146. 

2 Walcott, Notes on the Cambrian rocks of Pennsylvania and Maryland from the Susquehanna to the 
Potomac: Am. Jour. Sci., Vol. XLIV, Dec, 1892, p. 481. 

3 Keith, The geologic structure of the Blue Ridge in Maryland and Virginia: Am. Geologist, Dec, 
1892, Vol. X, No. 6, p. 365. 


sandstone" of Rogers, and are represented by him as interbedded with 
slates and occurring in a series of narrow anticlines. They form the 
u Mountain Creek rock" of Frazer and Lesley, mapped by them as 
Azoic and represented both in sections and text as underlying the! 
"chlorite schists' 1 and "bedded orthofelsites." Messrs. Geiger and 
Keith, 1 in their earlier investigation of the southward extension of; 
this sandstone in the Blue Ridge, placed it above the limestone of the 
Cumberland Valley, and, upon structural grounds, determined its age 
as Upper Silurian (Medina). The true age of these sediments has 
recently been determined beyond question, through the discovery of 
fossils, by Mr. Walcott. 2 No fossils were found in the quartzose con- 
glomerates and quartzites of the Monterey district (No. 2 of Mr. Wal- 
cott's section), but the interbedded argillaceous shales, of which men 
tion has been made, are found to the west of the Monterey district pass- 
ing beneath quartzite and other shales in which were discovered the 
remains of Scolithus linearis, Olenellus, and Camerella minor. These 
discoveries undoubtedly refer the conformably underlying Monterej 
quartzite to a Lower Cambrian age. 

In a recent section made in the light of Mr. Walcott's discoveries anc 
of his own subsequent discovery of fossils in the Blue Ridge, Mr. Keith 
gives the Monterey sediments a similar position (No. 4), describing then 
as a "massive white sandstone with bluish black bands, feldspathic, anc 
in places conglomeratic." Mr. Walcott calls them "a coarse-grainec 
quartzite, sometimes conglomeratic." 

The sections (PL V) through the Monterey district show the relative 
distribution and position of the three type rocks. It will be observed 
that the sandstone lies in a gentle syncline, or almost flat, and wholly 
above the eruptives. These observations made by the writer in the 
Monterey district and elsewhere in the South Mountain, accord with the 
observations of Mr. Keith 4 in Maryland. Owing to the entire absence 
of exposure, save at the tunnel, the minor folding, which is undoubtedly 
present at many other points, is not indicated in the sections. 

Mr. Keith 3 finds that in Maryland, as a result of faulting, the igneous 
rocks occasionally overlie the sandstone. This is nowhere the case 
in the Monterey district. The facts that no inclusions of sandstone 
were found in the volcanic breccia, and that fragments of the acid erup- 
tive occur in the quartzose conglomerate, suggest the superficial char- 
acter of the latter. 

Further indications of the same nature will be discussed in consider- 
ing the comparative age of the sedimentary and igneous rocks. 

1 11. 11. Geiger and Arthur Keith, The structure of the Blue Ridge near Harpers Ferry : Bull. Geol. 
Soc. Am., Vol. II, p. 163, pis. 4 and 5. 

2 C. D. Walcott, Notes on Camhrian rocks of Pennsylvania and Maryland: Am. Jour. Sci., Vol 
XLIV, 1892, p. 481. 

3 A. Keith, Geologic structure of Blue Ridjre in Ma^land and Virginia: Am. Geologist, Vol. X, 18£ 
p. 305. 

4 Geiger and Keith, loc. cit., p. 155, pis. 4 and 5. 
6 Loc. cit.,p. 365. 













>, ■ * K*^L-*8^* 




Economic value. — The susceptibility of these porphyries to a fine 
polish and their suitability for ornamental purposes have been remarked 
by many of those who have studied them. 

The earliest mention of this sort was made in 1822 by Dr. Hayden, 1 
who notes " handsome porphyry, Nicholson's Gap, Blue Ridge, Penn- 
sylvania, crystals red and distinct." 

Speaking of Oatoctin Mountain, the southern extremity of South 
Mountain in Maryland, Tyson says: 2 

Its porphyries and amygdaloids are deserving of the attention that I propose here- 
after to bestow upon them. Some of them will receive a beautiful polish, bat their 
hardness renders the process expensive. This can, however, be overcome by appro- 
priate machinery. 

Dr. Frazer 3 has, as already quoted, described the porphyries as 
" suitable for an ornamental building stone." 

Dr. Hunt 4 has also called attention to the fact that " these peculiar 
rocks, which make such a conspicuous figure in the South Mountain of 
Pennsylvania south of the Susquehanna, are of interest economically 
from the fact that they are in other regions the repositories of rich 
iron ores, and also because they afford ornamental porphyries of rare 
beauty, similar to those wrought in Elfdalen, in Sweden." 

The Tenth Census 5 reports that "it [the South Mountain porphyry] 
is well adapted to ornamental work, as it is rich in color, durable, and 
susceptible of a good polish, and in many cases could be obtained in 
abundant quantities. It has not yet been quarried for purposes of 


Areal distribution. — The basic eruptives occupy, in this district, an 
area fully twice as large as that covered by the acid eruptives consti- 
tuting the major part of the valleys, foothills, and mountain flanks. 

Character. — The rocks are massive, schistose, or slaty. They are 
usually conspicuously amygdaloidal, and associated with these amygda- 
loids are banded fine-grained schists, which have been considered altered 
accumulations of volcanic ash. In a section exposed on the Gettysburg 
Railroad there are alternating bands, from 2 to 3 feet wide, of a compact, 
fine-grained, epidotic rock which may also represent a basic volcanic 
ash. Bombs were found embedded in this epidotic rock. As with the 
acid rocks, there are accompanying basic breccias, though the latter are 
not abundant. The cementing material in every case was epidotic. 
An agglomerate formed of rounded fragments from an inch to 6 inches 
in diameter was also found. There was no opportunity for estimating 
the thickness of the accumulated basic flows. Wells have been bored 
in the basic rock to the depths of 55, 85, and 110 feet. 

1 H. H. Hayden, Mineralogical notes : Am. Jour. Sci. and Arts, Vol. V. 1822, p. 255. 

2 Tyson, First Annual Report, 1860, Appendix, p. 3. 

3 Frazer, Vol. CC, p. 285. 

4 Hunt, Proc. Am. Ass. Adv. Sci., 1876, p. 212. 

6 Tenth Census Report on the Building Stones of tbe United States, p. 168. 

. . : 



IZ! Cambrian Sandstone HUtyi..-.-.,. Formation. 


Volcanics (Pre 

Basic Ynlt.ti) 


The basic rock, by reason of its softer character, is more subject to 
alteration under dynamic action than are the hard acid rocks. The 
effect of this is seen in the almost universal schistosity of the basic 
rock. This metamorphism is accompanied by a correspondingly greater 
chemical alteration than is shown by the acid rocks. The alteration 
consists largely in the abundant development of epidote and chlorite, 
which gives to the rock its uniform green color and its popular name of 
" greenstone." 

Previous descriptions. — Dr. Hayden 1 notes the remarkable develop- 
ment of epidote as follows: 

Most beautiful epidote, with green and other shades of copper scattered in quartz. 
The blue is prevalent. Abundant in Blue Ridge. * * * 

Quartz and epidote. with green carbonate and red oxid of copper and native 
copper. Blue Ridge; abundant. 

The greenstones are locally known as the "copper rock," because of 
the copper ore which they carry. They are Rogers's "lower Primal 
slates," which "are highly indurated, and even decidedly crystalline, 
containing in some of their layers segregated specks, and even half- 
formed geodes [amygdules] of epidote and other minerals." "A highly 
altered, greenish slate." " Dark-green slate, with its epidote and 
white, intrusive quartz." Tyson describes them as " slates." Dr. Frazer 
calls them u chlorite schists," and Dr. Hunt terms them epidotic or 
chloritic rocks. Blandy approaches most nearly the present idea of 
their nature in describing them as "amygdaloidal trap." 


The workable deposits of ore in the South Mountain fall into two 
lasses : The limonite ores, which are deposited in Paleozoic sedimeut- 
iry rocks, and the copper ores, which are associated with pre-Paleozoic 
gneous rocks. 

Profitable limonite iron mines are being worked at various localities 
n the South Mountain, in the Cambrian sandstone, and along the con- 
tacts between it and the Silurian limestone. They are fully discussed 
n the Final Report of the Second Geological Survey of Pennsylvania, 
Vo\. I. 

One such ore bank occurs within the Monterey district, on its north- 
astern boundary, at the contact between Triassic limestone and the 
Jambrian sandstone, near Old Maria furnace. This bank is exhausted 
ind has not been worked for many years. 

An interesting occurrence of copper ore and native copper 2 is to be 
bund within the South Mountain chain, in a belt extending from a 
mint some 6 miles north of the State line, in a southwesterly direction, 

'Loc.cit.,1822, p. 255. 

2 Frazer, Copper ores of Pennsylvania: Polytechnic Review, Vol. Ill, 1887, No. 16, p. 159; An hypoth- 
sisof the structure of the copper belt of the South Mountain, pp. 82-85. Henderson, The copper 
eposits ot the South Mountain: Trans. Am. Inst. Min., Eng., Vol XII, pp. 85-90. 


to and beyond the Stale line into Maryland. This belt lies along tin 
contact between the acid and basic eruptives. 1 

The principal localities where shafts have been sunk within this bel 
are. the Snively copper mines, the Rnssel farm (the Bechtel shaft), tin 
Reed copper pits, the Bigham copper mine, and the Headlight mine 
(See map, PI. IIT.) To the east of this belt, on the road between Fair 
field and Fountaindale, a fifth old copper shaft is located. 

The Snively copper mine, which is a mile north of the Monterey dis 
trict, was not visited by the writer. 

Frazer reports that the copper is associated with quartz, epidotn 
rock, and azurite. 2 

At the Rnssel copper mine, which is located on the Rnssel farm, at tin 
fork of Copper linn, native copper occurs in quartz veins, associatec 
with calcite, in a very siliceous epidotic rock, an epidosite. The mate 
rial thrown out at the lower shaft was amygdaloidal greenstone; at tin 
upper or Bechtel shaft, an acid slate, which indicates that the coppe 
belt is along the contact between the two igneous rocks. 

The Reed copper pits lie on the north side of Toms Creek, at tin 
forking of the highroad near Spring Run. It has not been worked fo 
forty years, and there is nothing visible to indicate the character of th< 
copx^er occurrence. The dump pile shows scoriaceous greenstone: 
with malachite stains. Dr. Snively reports that native copper occur 
here in quartz veins. 

The excavation for copper ore on the Bigham property, which lie 
just northwest of the forking of Gladhills road, furnishes interesting 
material for the petrographer. It is here that acid amygdaloids havi 
been exposed in great perfection. They are abundantly stained witl 
malachite, azurite, or cuprite. Metallic copper occui\s in quartz vein 
traversing the amygdaloids, and in submacroscopic quantities in tin 
amygdules. In the latter case the copper is frequently surrounded ty 
zones of the oxide and the carbonate. An analysis of these rocks give 
4 per cent of copper. The Headlight mine is tunneled beneath the turn 
pike half a mile (eastward) below the Clermont House. Here the dumj 
pile shows only greenstones, more or less stained with copper carbonates 
This is also the case at the old copper shaft on the Fairfield road 
Asbestos in quartz is quite generally found at these copper mines. 

Throughout this belt the copper is evidently of secondary origin 
occurring in seams and amygdules, where it has been deposited iron 
solution. This occurrence of metallic copper in the igneous rocks o 
South Mountain is interesting because of its similarity to the Lak« 
Superior copper-ore deposits. Its associations are quite analogous 
In the Lake Superior region it occurs in veins, quartzose and epidotic 
in the open-textured amygdaloids (basic), and in interbedded con 

'Frazer says: "The ore belt lies in the orthofelsite which forms this portion of the chain." 
2 P, Frazer, Copper ores of Pennsylvania: Polytechnic lloview, Vol. Ill, 1877, Xo. 16, p. 170; also 
Appendix, Vol. CCC, Seconu Geol. Surv. Pa., p. 310. 


lulglom crates. In the South Mountain, in the absence of contemporaneous 
interbedded elastics, the occurrence is limited to the igneous rocks. 
The replacement of the feldspar by chlorite and epidote, and their 

I, replacement in turn by copper, characterizes the amygdaloids of the 
South Mountain also. 

The explanation of the precipitation of the copper is doubtless the 
same in both regions. 1 

A correlation of the South Mountain formations and the Keweena- 
^an copper- bearing rocks was suggested by Mr. Blandy in the article 

D jefore quoted. There is great petrographical similarity between the 
porphyrites and felsites and diabase porphyrites of the Keweenawnn 
series and their equivalents in the South Mountain, but it seems to the 

[ e writer unwise to parallelize on petrographical evidence the South 
Mountain igneous rocks and the Keweenawan of Lake Superior. All 

U jiorrelation upon such grounds, when applied to rocks widely separated, 
s well known to be untrustworthy. There has not as yet been found 
urfficient evidence to show to what horizon in the Algonkian series the 

t l jgneous rocks of South Mountain should be referred. 




That the Cambrian rocks do not underlie the " slates" and "ortho- 

li [elsites," as stated in the reports of the Pennsylvania surveys, is quite 

)lain, but whether the sediments are entirely subsequent to the 

gneous rocks or are in part, at least, contemporaneous, it is not so 

pasy to decide. 

Contacts between the sedimentary and igneous rocks are finely 
exposed at two localities. ' About halfway through the tunnel on the 
Gettysburg Uailroad the basic igneous rocks and the Cambrian rocks 
i Y lire in contact. Both formations dip gently to the southeast (±20°). 
|Che sandstone has become an indurated quartzite. Close to the green- 
„ g itone it has acquired a green color, due to the abundant development 
f chlorite. It has also become very schistose, and in fact might 
eadily be confused with the greenstone itself. Granulation, undu- 
atory extinction, and the obliteration of all direct evidence of clastic 
rigin, as revealed by the microscope, indicate the action of dynamic 
orce on the quartzite. The greenstone is slaty and too decayed at the 
01 1 tact for microscopic study. 

Southwest of Old Maria Furnace, in an abandoned cut on the Tape- 
worm Railroad, a great surface of red felsite 2 is partially exposed and 
artially overlain by the clastic formation. (PI. Yl.) Here again 
here is a gentle dip of both rocks to the southeast (i-15°). The sand- 

^umpelly, Geology of Michigan, Vol.1, Part III, p. 43. 

2 Felsito is used here as a field term for a compact, stony, nonporphyritic or inconspicuously por- 

lyritic volcanic rock. See pp. 37-38. 



stone has become a vitreous quartzite. The hard felsite does not sho 1 
alteration. That the igneous and sedimentary rocks have been sul 
jected to the same forces of folding is shown by the uniformity of then 
cleavage dips at both these localities. 

This conformity of structure planes, the absence of contact meta- 
morphism, and the evidences of dynamic action indicate planes of 
thrust rather than planes of original deposition or of subsequenl 
igneous intrusion. 


These two contacts do not decisively prove the younger age of t 
overlying sediments, although they very strongly indicate it. If th 
sandstone has been thrust over the lava from beneath, it would be ne 
essary to suppose an enormous amount of erosion to account for th 
entire absence of volcanic material above the sandstone. On the other 
hand, it is very easy to suppose that overlying sediments, on being 
subjected to pressure, were thrust up over the igneous rocks from the 
east. This explanation of the facts coincides with other evidence rel 
tive to the comparative age of the sedimentary and volcanic rocks. 

(1) Nowliere in the South Mountain has there been found a dike of 
the volcanic material in the sedimentary rock. Were the lavas more 
recent than the sediments, the former could hardly fail of tilling cracks 
in the latter. 

(2) No sedimentary beds have been found intercalated with the lava 

(3) There is no evidence of the alteration of the sedimentary rock 
by igneous contact. 

(4) The igneous rocks, in their fluxion and amygdaloidal structures, 
and in their accompanying pyroelastics and ash deposits, bear every 
evidence of being subaerial lava flows. 

(5) The presence of igneous fragments in the basal conglomerate of 
the sedimentary formation shows that erosion of the igneous rocks was 
taking place while the sediments were accumulating along their edges. 

While it is true that some erosion of the eruptives must have taken 
place before and during the deposition of the Cambrian sediments, 
there is no evidence of extended erosion; indeed, the character of sur- 
face flows, which the lavas retain so conspicuously, would incline one to 
suppose that no great load of material has been removed. Mr. Keith's 
conception that the granite which in Maryland is intruded into the 
"diabase" (greenstone) is younger than the volcanics necessitates 
an immense original thickness of the diabase flows and an extended 
subsequent erosion to expose the plutonic eruptive. In this respect 
his argument for the comparative age of the deep-seated and surface 
igneous rocks seems at fault 1 . Such unlimited erosion is unsupported 
by field observations in the South Mountain. 

1 Geologic structure of Blue Ridge iu Maryland aud Virginia: Am. Geologist, Vol. X, 1892, p. 368. 



Felsite. Sandstone. 






The relative age of the acid and basic volcanics is an interesting 
question, a question to which it is not possible with our present knowl- 
edge to give an entirely satisfactory answer. 

On the one hand we find the acid rocks occupying, as a rule, the 
lower altitudes, and even overlain by the basic rock. Occurrences of 
the latter nature are located as follows: 

(1) At the junction of Gladhill's road and the Fountaindale turn- 
pike, in the northeast angle of the roads, a slight excavation exposes 
an outcrop of gray, slaty felsite overlain by greenstone. 

(2) Not far from the source of Minie Branch there is exposed by the 
cutting of the stream a gray felsite, overlain by greenstone. 

(3) On the road from Fountaindale to Fairfield, southeast of Jacks 
Mountain, a much-altered felsite is overlain by greenstone. 

(4) Lastly, on the Fountaindale turnpike, about half a mile above the 
Emmetsburg tollgate, the same superposition is exposed by the roadside. 

The only published statement of opinion as to the relative age of the 
volcanics has been made by Mr. Keith. 1 From his observations of the 
relative positions of the acid and basic rocks in Maryland, Mr. Keith 
expresses himself as confident that the " quartz-porphyry underlies the 

On the other hand, over against this somewhat meager proof must 
be considered a few facts of another sort. Professor Williams reports 
that a position of the acid and basic rocks the reverse of that which has 
been described in the Monterey district occurs north of that district. 
Throughout the northern portion of the range the acid eruptives, which 
abound almost to the exclusion of the basic eruptives, form the moun- 
tains, while the latter occupy the valleys. 

We must, moreover, bear in mind that the principles of stratigraphy 
employed in determining the age of sedimentary rocks may prove very 
misleading if applied to igneous rocks. The younger lava may be 
found overlain by the materials of an earlier eruption where the 
former, in coming to the surface, breaks through the older formation. 
The latest eruption may fill the depressions. 

Where two different lavas occur on the same level, as is the case 
south of the Clermont House, the lava (in this case the acid lava) occur- 
ring as a narrow strip inclosed by the other might be an intrusive 
eruptive, and thus younger than the inclosing basic lava. 


With these principles in mind, with conflicting field evidence, and in 
the absence of genuine dikes of either acid or basic character, the data 
for the determination of the comparative age of the acid and basic 

i Loc. cit., p. 367. 


volcanics are not sufficient. In all probability there were several 
sources of lava flow. 

The southern vents furnished the great areas of basic lava (green- 
stones) in Maryland and southern Pennsylvania, while the northern 
vents poured out the enormous acid flows. 

In the Monterey district the two lavas are mingled, and apparently 
the basic flow was preceded by the acid flow. 

Thus the facts observed by Professor Williams in the north, Mr. 
Keith in the south, and the writer in the Monterey district, may be 
brought into accord. 


Three types of rocks are present in the South Mountain : (1) Siliceous 
sedimentary rocks of Lower Cambrian age, overlying with structural 
conformity and stratigraphical unconformity igneous rocks. These 
igneous rocks are surface flows of (2) an acid and (3) a basic constitu- 
tion. They are probably pre-Oambrian, and lithologically resemble the 
Keweenawan copper-bearing rocks of Lake Superior. 

There is not sufficient evidence to decide their comparative age, 
though in the Monterey district field observations, on the whole, indi- 
cate that the acid rocks are the older. 

The subaerial flows were subjected to a limited erosion before the 
sediments were deposited and while they were being deposited, so that 
the region must have possessed some elevation at that time. 

The intense dynamic action shown by the igneous rocks occurred 
after the deposition of the sediments. Since the sediments were laid 
down the whole region has been subjected to lateral pressure (at the 
time of the Appalachian uplift), whereby the igneous rocks were cleaved 
and sheared and the sedimentary formation was thrust up over them 
from the east — where it has been largely eroded, occurring now only 
sporadically — and the whole region was elevated. That erosion has 
removed a great thickness of material since this elevation is indicated 
by the cleavage dips of the igneous rocks. 



The sedimentary formation exhibits two marked phases, the con- 
glomeratic and the qnartzose. Its lowest member is conglomeratic. 

These conglomerates are frequently slaty, through the development 
of more or less sericite. They contain quartz pebbles from an inch to 
an inch and a half in length, and sometimes show included fragments 
of quartz-porphyry and of a green slate. 

From a conglomeratic character the sediments pass through a coarse 
sandstone into a compact quartzite exhibiting under the microscope the 
characteristics of a r eery stalli zed clastic. 

The qnartzose sandstone is exposed in great masses on the flanks and 
summit of Monterey Peak, on Haycock Summit, at the Gladhills Switch 
on the Gettysburg Railroad and at many points along the railroad 
as it skirts the east side of Jacks Mountain, and in massive pinnacles 
along the crest of that mountain. That the sandstone has been greatly 
fissured and broken is indicated by the conspicuous and frequent quartz 
veining, but metamorphism, save in a few cases, has been limited to 
the formation of a vitreous quartzite by the deposition of a siliceous 
cement. The original stratification planes are usually preserved, and 
cross bedding is frequently conspicuous. A secondary cleavage is also 
very marked, so much so as, in the absence of well-defined stratifica- 
tion planes, to be mistaken for the bedding. 



Four specimens of typical quartzite were studied in the thin section. 
They were obtained from widely separated localities, and range in color 
from gray to a light green. Two of them illustrate the enlargement of 
quartz fragments and the genesis of a quartzite as perfectly as do any 
of the Lake Superior sandstones. 1 (PI. XV, a.) 

1 Qnartz enlargements were first described by Tornebohm, subsequently by Sorby, Young, Irving 
and Van Hise, Bonney, Phillips, and Iddings: 

A. E. Tornebohm, Ein Eeitrag zur Frago der Quartzbildung : Geol. Foren. Stockh. 1876, Vol. Ill, 
p. 35. Kef. Neues Jahrbueb fur Mineral., 1877, p. 210. 

II. Clifton Sorby: Anniversary Address, Quart. Jour. Geol. Soe. London, Vol. XXXVI, 1880, p. 62. 

A. A. Young: Am. Jour. Sci., A r ol. XXII, July, 1881. 

It. I). Irving: Am. Jour. Sci., Vol. XXV, June, 1883. 

E. D. Irving and C. R. Van Hise: Bull. U. S. Geol. Survey No. 8, 1884. 

T. G. Bonney and J. A. Phillips: Quart. Jour. Geol. Soc. London, Vol. XXXIX, 1883, p. 19. 

E. U. Irving and C. E. Van Hise: The Penokee iron-bearing series of Michigan and Wisconsin; 
Tenth Ann. Kept. U. S. Geol. Survey (1888-89), p. 375, I']. XXVII, fig. 2. 

J. P. Iddings: Mon. TJ. S, Geol. Survey, Vol. XX, 1892, pp. 346-347. 



The original grains are conspicuously outlined by a rim of iron oxide 
and surrounded by interlocking areas which are optically continuous 
with the inclosed grain. Rarely the grain itself is composed of two 
differently oriented areas, in which case the enlargement is also com- 
posed of two different areas oriented with the two portions of the grain. 

These same specimens show feldspar grains, usually with the " grid- 
iron" structure of microcline. Occasionally a hornblende fragment 
is also present. The rock is evidently derived from granitic debris 
and might more accurately be called arkose. The fresh character of 
the fragments is in general noteworthy and suggests a source not far 

The best examples of the vitreous quartzite, the extreme phase of 
metamorphism in the sediments, are to be found in the tunnel at and 
in the neighborhood of the contact with the greenstone, and on the 
unfinished Tapeworm Railroad southwest of the Old Maria Furnace at 
the similar contact of the sandstone and felsite. At both of these 
localities the rock shows macroscopically and microscopically the effect 
of dynamic action. Of two specimens from the first locality, which 
has already been described in sufficient detail on page 27, one is a pure 
quartzite with prominent cleavage, on the surface of which iron rust is 
deposited. The thin section shows a quartzite from which every ves- 
tige of the original waterworn grains has been obliterated. The rock 
consists of interlocking areas of quartz and some interstitial material. 
This is a tine aggregate of quartz grains, and may be a remnant of the 
granulation which must have occurred as a result of the movement of 
the grains against one another. 

A similar phenomenon in the sandstone of Sugar Loaf Mountain has 
been figured by Dr. Keyes. 1 

Another specimen from the same locality, but nearer the greenstone, 
shows a rock which, because of the shearing and chloritization that it 
has undergone, very closely resembles a chlorite schist. Its clastic 
character, however, becomes quite evident under microscopic examina- 
tion. This shows it to be a siliceous rock, greatly sheared and altered 
in character by the formation of sericite and chlorite. It is the only 
section of quartzite in which chlorite has been observed. Its close 
proximity to the greenstone, in which that mineral is so abundantly 
developed, suggests a probable source of the magnesia, iron, and alu- 
mina. Tourmaline, zircon, magnetite, and feldspar are also present in 
this rock. 

The quartzite at the felsite contact is similar, both in the hand speci- 
men and in the thin section, to the first of the two specimens just de- 
scribed. Ko trace of the original grain remains. Undulatory extinction 
is pronounced and recrystallization complete. Specimens obtained close 
to the acid igneous rock differ only in the presence of a large amount of 

1 C. P. Keyes, A geological section across the Piedmont Plateau in Maryland : Bull. Geol. Soc. America^ 
Vol. II, 1891, p. 321. 

bascom.] SLATES. 33 

red iron oxide. The quartz crystals are frequently cracked, testifying 
to more intense dynamic action. The extreme induration at both these 
contacts is confined to a selvage of the sandstone. 

In a cut on the Gettysburg- Railroad, southwest of the Old Maria Fur- 
nace, the sedimentary rock is locally of a different character from that 
which has been described. It resembles the second specimen described 
from the contact in the tunnel more closely than any other of the elas- 
tics. Shearing has been accompanied by the development of sericite 
and chlorite, producing' a soft, green, slaty rock. The chloritic areas 
have usually clearly denned boundaries, and possibly represent former 
hornblende grains. Zircon is present. The major part of the rock con- 
sists of quartz grains, with undulatory extinction. 

Another local alteration of the clastic rock is to be found on the hill, 
tops southeast of Jacks Mountain. Here it appears as a yellowish 
schistose rock. The thin section is characterized by the development of 
a large amount of sericite and granular epidote. The sericitic scales 
show a parallel arrangement, which is the product of shearing. The 
color of the rock is due largely to an iron hydroxide. 

Among the sedimentary rocks within the Monterey district there 
rarely occurs, associated with the sandstone, an argillaceous slate. 
This is exposed just north of Jacks Mountain Station. It is the only 
representative in the Monterey district of the interbedded slates occur- 
ring west of that district. It is silky, pearl-gray, crinkled, and cleaves 
readily into slabs. 

The thin section shows that the rock has been somewhat recrystal- 
lized, though its clastic origin is still apparent in the angular shape of 
its quartz fragments. Quartz and sericite are the principal constitu- 
ents of the rock. The other constituents are magnetite, hematite, and 
a gray cloudy aggregate which, under the highest power, obscurely 
suggests leucoxene and granular epidote. 


The analysis of the quartzite given below shows proportions which 
would be possible only in a derivative rock where the exact chemical 
composition is a matter of accident and subject to no fixed laws. 

The silica percentage is disproportionately high for any but a clastic 
rock. The alkalies indicate the presence of a considerable amount of 
either orthoclase feldspar or sericite, probably the latter. The alumina 
and lime percentages denote the presence of epidote. 
Bull. 136 3 


Analysis of quart zite. 1 

[bull. 136. 

Per cent. 


Si0 2 (quartz) .... 
KiO., (Combined) . 

84. 130 








1.1 CO 



100. 680 

I K 2 

Na 2 .. 

P./), . . 

ALO { . . . 


Fe 2 3 . . . 


The Cambrian sediments of tlie Monterey district may be classified 
as conglomerate, sandstone, qnartzite, and slate. 

The first two of these phases represent original sedimentary deposits, 
of which the latter has been locally metamorphosed by induration into 
a qnartzite. The slates are in some cases from deposits of a more 
finely divided material, and in other cases they are the product of shear- 

There has been a limited and local introduction of new material, 
which shows itself in the production of chlorite and epidote. In every 
case the clastic character of the rock remains indubitable. 

1 Specimen No. 1157, 1| miles S. of W. of the burned sawmill on the Conocochoa^ue. This analysis 
was made by Dr. F. A. Genth, formerly of Pennsylvania University, for the Second Geological Sur- 
vey of Pennsylvania, Vol. CCC, Second Geol. Snrv. Pa. 




The question of the nomenclature of the acid volcauics has proved 
to be one of considerable interest and importance. In the discussion 
of suitable terms for the description of these rocks, it will be necessary 
to anticipate, in some degree, the results of their microscopic study. 
A variety of names have been applied by petrographers to the acid 
type of the older volcanic rocks. Under the general group of quartz- 
porphyries, Rosenbusch classifies them as microgranites, with a micro- 
granitic groundmass; granophyres, with a micropegmatic groundmass; 
felsophyres, with a microfelsitic base; and vitrophyres (including pitch- 
stones and pitchstone-porphyries), with a vitreous base. Fouque and 
Levy employ micro granitite, micropegmatite, and porphyre petrosiliceux 
as corresponding terms. British petrographers have described these 
acid rocks under the terms Jiornstoxes, claystones and clay stone-por- 
phyries, felsites, quartz- felsites, and felsite-poryhyries, agreeing in this 
respect with the older German usage, when they have not followed 
Eosenbusch. In America both German and English usages have been 
followed, with more or less confusing results. In the nomenclature of 
the South Mountain rocks an effort has been made to avoid such con- 
fusion and to use such a term or terms as shall accurately characterize 
them and all similar rocks. 

Among the acid eruptives of South Mountain are typical represen- 
tations of Bosenbusch's quartz-porphyries. Closely associated with 
them and impossible of separation by any sharp line of demarcation are 
acid rocks with every structural characteristic of modern lavas. 

Although possessing some characteristics in common with the felso- 
pliyres, these ancient prototypes of the rhyolite can not be included 
under that term, since they have a holociystalline groundmass. Inso- 
much as many of the English felsites have been shown by liutley, 
All port, Cole, and Bonney to be devitrified obsidians and pitch- 
stones, and thus, like the American rocks, representatives of the glassy 
lavas of pre-Tertiary times, these South Mountain lavas might, with 
some propriety, be termed felsites. Strict consistency would then coin- 
pel the replacement of the term quartz-porphyry by the more limited 
and less well-established term quartz-felsite, which was felt to be a dis- 
advantage. Moreover, the distinction between the rhyolitic lavas of 



South Mountain and the typical quartz -pophyries is not one of the 
absence or presence of phenocrysts. Thus the term felsite would be 
forced to cover not only nonporphyritic acid rocks, but also conspicu- 
ously porphyritic rocks. Finally, felsite, though useful as a field name, 
may well be objected to as an inaccurate petrographical term. 

A brief history of the word felsite and its synonyms in different coun- 
tries (petrosilex, eurite, and halleninta,) will serve to illustrate the 
unfitness of it and these allied terms for exact petrographical usage. 

"Felsit" was introduced into German petrographical nomenclature 
by Gerhard 1 in 1814, who applied the term to a matrix which he claimed 
was common to all feldspar, claystone, and hornstone porphyries. This 
matrix was a compact homogeneous aggregate of feldspar and quartz, 
supposed by Gerhard to be compact feldspar. 

In 1858 Naumann' 2 adopted Gerhard's word for the feldspar-quartz 
groundmass, and called all porphyries with such a base felsite-porphy- 
ries. This term has since been applied by Tschermak to porphyries 
without quartz phenocrysts (orthofelsites, orthophyres). 

The more accurate German usage of the present day discards felsite 
save as a macroscopic term for an unresolvable porphyry base, 3 while 
"felsitfels" is used if, in the absence of phenocrysts, the rock is com- 
posed of this base only. 4 

This same quartz-feldspar mosaic, confused with compact feldspar, 
was distinguished as petrosilex by Wallerius 5 as early as 1747, and 
subsequently by Dolomieu. This term was also early used by Bron- 
gniart. In 1819 the same groundmass was called eurite by Daubisson * 
because of its fusibility. Michel-Levy 7 uses the term petrosilex, but 
with a more limited meaning. He defines it as essentially synonymous 
with Eosenbusch's microfelsite, making "porphyre petrosiliceux" equiv- 
alent to felsophyre. Petrosilex is, however, generally used by Levy and 
other French petrographers in a more or less loose and vague way, to 
cover a crystalline or cryptocrystalline quartz -feldspar aggregate or a 
partially amorphous siliceous feldspathic magma. 

In Sweden, the fine-grained, apparently homogeneous acid rocks 
consisting essentially of quartz and feldspar and rarely porphyritic, 
are called halleninta. 8 They may be of aqueous or of igneous origin. 

The early meaning of felsite in England was quite similar to that 
given it on the Continent. According to Pinkerton 9 the term was 

'Beitriige zur Geschichte des Weissteins ties Felsit und anderer verwaiidten Arten: Abhandl. 
K. Akad. Wiss. zu Berlin, 1814, 1815, pp. 18-2G. 

2 Lebrbuch der Geognosie, 2d ed., Vol. I, 1858, p. 597. 

3 Rosenbuseh, Mikro. Pliys., etc., 2d ed., Vol. II, p. 373. 

4 Rosenbusch, loc. cit., p. 351. 

5 Systematica Mineralogicum, 1847, French translation, 2 vols., Paris, 1853. 

G Traite do geognosie, lsted., Vol. I, 1819, p. 112. 

7 Structures et classification des roches eruptives, p. 17. 

8 Justus Roth, Allgeraeine und chemiscbe Geologie, Vol. II, p. 494. F. Zirkel, lyehrbucb der Pet> 
rographie, Vol. I, p. 564. 

9 T. Pinkerton, Petralogy, A Treatise on Rocks, Vol, I, 1811, p. 161. 

bascom.] NOMENCLATURE. 37 

introduced in 1794 by Kirwan l for compact feldspar. Current usage as 
defined by Teall 2 applies the term to u compact stony rocks, the min- 
eral composition of which can not be ascertained by examination with 
the naked eye or with the lens. * * * These rocks are anhydrous 
(or nearly so), and except in this respect agree in composition with the 
acid glassy lavas." 

Dana 3 (as an exponent of American usage) defines "felsyte," a rock, 
as " compact orthoclase with often some quartz intimately mixed, fine 
granular to flint-like in fracture, * * * both metamorphic and 
eruptive." The mineral he defines as follows : " Felsite is compact, 
uncleavable orthoclase, having the texture of jasper or flint, which it 
much resembles. It often contains some disseminated silica. It is 
distinguished from flint or jasper by its fusibility. * * * It is the 
base of much red porphyry." This is substantially the " felsitfels " and 
r felsit" of Eosenbusch. 

In short, " felsite" has been used to describe an acid base, unresolvable 
by the naked eye, and once supposed to be a single mineral. With the 
Introduction of the microscope this macro" felsitic " base was resolved 
into the microgranitic, micropegmatitic, and microfelsitic groundmass, 
the point of ignorance having been shifted from the felsitic base, 
macroscopically unresolvable, to the microfelsitic base, which is micro- 
scopically unresolvable. 4 

On the Continent " felsite" has practically been replaced by these 
terms. British and American petrographers retain it as a useful field 
name for rocks formed of this macroscopically unresolvable base with- 
out phenocrysts or with inconspicuous phenocrysts. In this sense the 
word will be used, when used at all, in this paper. 

It is very generally recognized that structural features are not con- 
ditioned by the geological age of rocks, but are a function of the condi- 
tions of consolidation. That these conditions, while very varied and 
complex in any geological period, have not essentially altered since 
Paleozoic times, has been shown to be the case by some of the most 
able observers. 5 

With this recognition has come the growing conviction among petro- 
graphers that mere age should be eliminated as a factor in geolog- 
ical nomenclature. While this is true, it is felt on the other hand 
that the rock name should show some recognition of the alteration 

'Richard Kirwan, Elements of Mineralogy. 

S J. J. Harris Teall, British Petrography, p. 291. 

:< J. D.Dana, Manual of Mineralogy and Lithology, 3d ed.. 1883, pp. 280, 442. Manual of Geology, 
3ded., 1879, pp.73, 77. 

"Rosenbusch does not assent to this interpretation of microfelsite, hut regards it as a definito 
chemical compound allied to feldspar, just as felsite was once regarded. 

! > Teall, Address of the Pres. of the Geol. Soc, of the British Ass. Adv. Sci., 1893. 

Judd, On the gahhros, dolerites, and basalts of Tertiary age in Scotland and Ireland: Quart. Jour. 
Geol. Soc., London, Vol. XLII, 1886, pp. 49-97. 

Allport, Tertiary and Paleozoic trap rocks: Geol. Mag., London, 1873, p. 196. British Carboniferous 
Dolerites: Quart, Jour. Geol. Soc, London, Vol. XXX, 1874, pp. 529-567. 

Iddings and Hague, On the development of crystali/.ation in the igneous rocks of Washoe, Nev., 
with notes on the geology of the district . Bull. U. S. Geol. Survey No. 17, 1885 


which the rock has undergone subsequent to its solidification. If at 
the time of its solidification the rock presented the features of a 
rhyolite, as is believed to have been the case with much of the South 
Mountain acid lava, but since that time has become holocrystalline, 
both these facts — its original character and its present character — 
should be recognized in the name. It is believed that this result may- 
be secured by the retention of such well-established names as rhyolite, 
obsidian, trachyte, etc., preceded by a prefix which shall have such a 
signification as will indicate the altered character of the rock. The 
prepositions meta, cpi, and apo all indicate, as prefixes, some sort of an 
alteration. Their exact force has been thus defined by Professor Gil- 
dersleeve. Meta indicates change of any sort, the nature of the 
change not specified. This accords with the use of the prefix by Dana 
in such terms as "metadiorite" and " metadiabase." These terms have 
been recently revived to designate " rocks now similar in mineralogical 
composition and structure to certain igneous rocks, but derived by 
metamorphism from something else." 1 

Upi signifies the production of one mineral out of and upon another. 
This prefix has not been much used. We find it in such terms as epi- 
diorite, epigenetic hornblende, and epistilbite. Apo may properly be 
used to indicate the derivation of one rock from another by some spe- 
cific alteration. 

If, therefore, we decide to employ this prefix to indicate the specific 
alteration known as devitrification (Entglasung) we may obtain, by com- 
pounding it with the names of the corresponding glassy rocks, a set of 
useful and thoroughly descriptive terms like aporhyolite, apoperlite, 
apobsidian, etc., as to the exact meaning of which there can be no doubt. 

In accordance with this usage, it is proposed in this paper to call all 
the acid volcanic rocks the structures of which prove them to have once 
been glassy, aporhyolites, while such as were originally noncrystalline, 
or whose original character is in doubt, will be termed quartz-porphyries. 

The writer feels that the introduction of a new name into petrograph- 
ical literature is to be deplored unless it can be shown that the name is 
formulated in accordance with certain well-defined principles. A good 
rock name should express composition, original structure, and as far as 
possible the process of alteration, if alteration has occurred. It is 
thought that aporhyolite and the suggested series of similarly formed 
terms meet these requirements. They are therefore adopted as prefer- 
able to any in present use. 2 

1 Whitman Cross, "On a series of peculiar schists near Salida, Colo.: Proc. Colo. Sci. Soc, Jan., 
1893, p. G. 

'-Since the ahove was written the term eorhyolite has heen proposed hy Dr. Nordenskjiild to cover 
ancient acid volcanics identical with those of the South Mountain. In a review of Dr. Nordenskjold's 
able paper, "Ueber archseisehe Ergussgesteine aus Smaland " (Bull. Geol. Instit. Upsala No. 2, vol. 1, 
1893), the writer has discussed the disadvantages that attend the use of any term or series of terms 
which carry with them the idea of age. The reader is referred to this review, which appeared in the 
American Geologist for March, 1896, pp. 179-184, for a fuller statement of the writer's idea of the rela- 
tion existing between devitrification and age of volcanics. 


It is a question whether it is always possible to distinguish between 
a primary and a secondary crystalline groundmass, and no attempt has 
been made to draw a sharp line between the quartz-porphyries and the 
aporhyohtes. In the absence of some of the more marked structures 
of a glass the presence of a secondary crystalline structure has not 
been considered sufficient evidence for the completely secondary char- 
acter of the crystallization. It is very probable that while a large 
portion of the lava flow consolidated as a glass, much of the lava solidi- 
fied at a sufficient depth to have secured a noncrystalline groundmass. 


A deep-red porphyry outcrops along the turnpike leading from the 
west to Monterey Station, and covers a small area north of the high- 
road. 1 One mile north of this, in the neighborhood of u Guni Spring," 
there are several limited areas of blue, purple, and brick-red porphyries. 
About 4 miles northeast, on the "Old Furnace Koad," by the unfinished 
viaduct, is a large area of dark-blue porphyry, passing toward the 
southwest by insensible gradations into typical aporhyohtes. 

These are the more important areas of porphyries. There are other 
porphyries so closely associated with the devitrified glassy lava that 
they will not be separately located. 


The beauty and variety of color of these porphyries have already 
been noted. The phenocrysts are not large (5 to 11 mm long), but are 
conspicuous against the dark or brilliantly colored matrix. The ortho- 
clase phenocrysts are opaque white, pink, or brick-red. They usually 
possess a well-dehned crystalline outline and show twinning in the hand 
specimen. The quartzes are colorless or a rich wine red. The fracture 
is conchoidal and there is a less-marked tendency to cleavage than in 
the aporhyolites. 

The porphyries show a reddish-brown color on the weathered surface, 
and on decomposing form a red earth. The presence of manganese is 
abundantly exhibited in dendritic markings on the cleavage surfaces. 
This is especially true in the case of the slates developed from the por- 
phyries. (PI. XIV.) 



Feldspar. — The porphyritical feldspars are more abundant than the 
quartz phenocrysts, and are remarkably fresh and unaltered. That 
they belong to the group of alkali feldspars, and that both monoclinic 
and triclinic varieties are uudoubtedly present, are indicated by chemi- 
cal analysis (given on page 01), by their specific gravity, and by their 

1 See the geological map of the Mouterey district accompanying this bulletin, PL 111. 


optical properties. Feldspar crystals were separated from specimens 
of the porphyry taken from different localities and their specific gravity 
was determined. The range was from 2.G to 2.62. On account of the 
presence m the heavier feldspars of minute inclusions of piedmontite, 
the lowest specific gravity was considered to represent the purest 

Twinning is very common, according to either the Carlsbad or Mane- 
bacher (periclme) laws. PI. XV, b, shows a Manebacher penetration twin 
of feldspar. The section is nearly parallel to the clinopinacoid, as the 
following observations indicate: The axis of least elasticity makes an 
angle with the twinning line (rhombic section) of about 15 degrees on 
one side and 21 degrees on the other; the obtuse positive bisectrix 
emerges. There is a slight trace of a basal cleavage and a microper- 
thitic intergrowth parallel to 6. The position of the plane of the optic 
axes, the distribution of the axes of elasticity, and the specific gravity 
all indicate that the crystal is the triclinic soda feldspar anorthoclase. 

The resemblance to this of the other feldspar phenocrysts makes it 
probable that this is the prevailing feldspar. Other sections show the 
albitic intergrowth (microperthite) developed in a pronounced manner 
(PI. XVI, a). Every gradation of this structure is present, from the 
microperthitic to the cryptoperthitic. Sometimes the perthitic growth 
does not persist throughout the crystal, but is present in an incipient 
stage along its edge. 

The feldspars are often cracked and drawn apart in the direction of 
the schistosity of the rock, and the cracks cemented with sericite scales 
whose parallel arrangement conditions the schistosity (PI. XVI, b). 
This is most striking in porphyry obtained 10 feet below the surface, 
which shows an abrupt passage into a sericite-schist. The phenocrysts 
are crushed and pulled apart. This action had been accompanied by 
an abundant development of sericite. Some of the phenocrysts pos- 
sess in the hand specimen a red color, which is due to a fine admixture 
of red iron oxide. These phenocrysts frequently prove, from a test of 
their optical properties, to be orthoclase. 

The alteration of the feldspars to epidote may indicate the pres- 
ence of lime in the feldspar. Undoubtedly, however, much of the lime 
is of secondary origin. This alteration of feldspar to epidote seems 
to be a direct one. There is no intermediate kaolin stage, such as 
observed by Eutley. ! Accompanying the epidote is granular quartz. 
These two alteration products usually occupy the center of the crystal 
and are surrounded by a rim of unaltered feldspar. 

Brilliantly colored piedmontite frequently fills cavities in the feldspar 
crystals. This mineral is often surrounded by a rim of epidote. 

Quartz. — The bipyrainidal or rounded quartz phenocrysts are remark- 
able only for their undulatory extinction and the cracks by which strain 

• Rutley, On some perlitic felsites and on the possible origin of some epidosites: Quart. Jour. 
Geol. Soc. London, Vol. XLIV, 1888, pp. 741-742. 


has been relieved. They are fresh and show characteristic ernbayments 
and inclusions. Like the orthoclase, they are sometimes reddened by 
inclusions of hematite, or are more rarely given a rich wine-red color 
by inclusions of piedinoiitite. 


The noncrystalline groundmass of the porphyries consists essentially 
of quartz and feldspar. These minerals form either a finely micro- 
granitic mosaic or a structure which becomes of considerable interest 
in connection with the question of the original character of the crys- 

In the first case there is no reason per se for supposing the crystal- 
lization to be other than primary. It is only as the crystallization 
is associated with secondary structures, or structures testifying to a 
primitive glassy condition, that the presumption favors its secondary 
character. When the crystallization is microgranitic and j)resumably 
primary, the rock is a genuine quartz-porphyry. 

The structure alluded to, which occasionally replaces the quartz-feld- 
spar mosaic in the porphyries, is the micropoikilitic, 1 where all the quartz 
of the groundmass has crystallized in irregular areas inclosing the feld- 
spar as lath-shaped microliths. These feldspar microliths are oriented 
quite independently of one another. This structure characterizes the 
aporhyolites, and the question of its primary or secondary character 
will be fully discussed in connection with these rocks. 

The alteration of the groundmass to sericite and the formation of a 
sericite schist will be discussed in connection with the acid slates. 


Piedmontite (manganese epidote) is the most remarkable of the acces- 
sory constituents. It is not only disseminated in microscopic quan- 
tities through the deep-red porphyry, but it also occurs in macroscopic 
masses, as a radiating aggregate, filling veins and cavities in the red 
felsite or replacing spherulitic crystallization. 

Two localities in the Monterey district furnish a great abundance of 
the piedmontite: the southwest flank of Pine Mountain, 1 mile north- 
east of Monterey station, and the hillside south of the Clermont House, 
between the turnpike and Minie Branch. Microscopic sections of the 
aggregate show brilliantly colored needles of piedmontite intergrown 
with clear quartz. (PI. XXV, b.) This intergrowth with quartz is also 
marked in the hand specimen. The radiating needles have the appear- 
ance of being broken, stretched apart, and the spaces filled with sec 
ondary quartz. The only other locality where piedmontite was found 
in macroscopic quantities is in the Buchanan Valley, 2 miles north of 

'G. H. Williams, On the uso of the terms poikilitic and micropoikilitic iu petrography: Jour, of 
Geol., Vol. I (No. 2, Feh.-March, 1893), pp. 170-179. 


the Chamber sburg turnpike, and one-eighth mile west of Musser's store. 
Pied mori tit e occurs here in cavities associated with sclieelitc.' 

Epidote is also abundantly present in the porphyries, passing by 
gradations of rose colored epidote to the deep carmine of the piecl- 
montite. Both of these silicates are undoubtedly secondary products. 

Innumerable black globulitesof manganese oxide and black and red 
iron oxide crowd the groundmass. In their arrangement they show 
rluxiou structure, or form meshes inclosing the quartz areas which 
constitute a part of the micropoikilitic structure. This arrangement is 
sometimes so marked as to be conspicuous in the hand specimen, giving 
the rock a mottled appearance. Crystals of zircon are not infrequently 

A study of the quartz porphyries of the South Mountain shows us 
that they are present there in characteristic development, and offer no 
marked variations from the normal type described from other localities. 
For this reason a brief space only has been devoted to them. 


There remains to be described a large and important portion of the 
acid rocks of the Monterey district. These will be termed aporhyolites, 
for reasons already suggested and to be more fully discussed in the 

The localities colored red upon the geological map of the Monterey 
district (PI. Ill) are aporhyolitic areas associated, in the cases already 
mentioned, with quartz-porphyries. The largest area begins just south 
of the Bigham copper mine, and, widening toward the north, extends 
far beyond the Monterey district. This area furnishes most of the 
spherulitic structures. A small detached area south of the Clermont 
House furnishes slaty and spherulitic aporhyolites. These aporhyolites 
and those of the four detached areas near the Maryland State line, 
separated from each other by a distance of a mile to a mile aud a half, 
show some marked points of resemblance, to which attention will be 
called later. 

The areas covering the foothills southeast of Jacks Mountain show 
an altered aporhyolite. There are several small areas of red aporhyolite 
on the Gladhill road leading to the Bigham property. At the forking 
of this road with the Fountaindale turnpike occurs the aporhyolite slate 
mentioned on page 29. 


The aporhyolites have about the same range in color as the porphy- 
ries, varying from light bluish-gray, or buff, to many shades of red 
and purple. While usually compact and always fine-grained, they are 

1 G. U. Williams Piedmontite and scheelite from the ancient rhyolite of South Mountain, Pennsyl- 
vania. Am. Jour. Sci., Vol. XLVI, 3d series, July, 1893, pp. 50-57. 



Four-fifths natural size. The specimen is deep red, with lines of flow in pink. 

bascom.] APORHYOLTTES. 43 

also sometimes very vesicular. In the latter case the amygdules of 
dark-green epidote and clear quartz, elongated by fluxion and con- 
spicuous against a pale-pink background, render the rock strikingly 
handsome. Phenocrysts are usually present, but generally incon- 

Delicate lines of flow structure, which are brought out in great 
detail by weathering or are painted in rich colors on the material 
washed by the mountain brooks, are another marked feature of the 
aporhyolites. These liow lines are frequently very sinuous, showing con- 
tortion and crumpling of the lava. (Pis. VII and VIII.) The tlowage 
is often emphasized by the mingling of two contrasting magmas, form- 
ing taxites of either a> eutaxitic or an ataxitic character. A fuller 
description of these taxites is given on page' 57. 

Another characteristic which these aporhyolites possess in common 
with their modern analogues is the spherulitic structure. Spherulites 
are rarely, if ever, altogether absent, and in some localities they are 
crowded so close together as to constitute the major part of the rock 
mass. They range in size from microscopic dimensions to those of a 
butternut. Where there is no regularity of arrangement and they are 
brought out in relief by weathering, the rock has a superficial resem- 
blance to a conglomerate composed of rounded pebbles of remarkably 
uniform size (BB shot) and shape. (PI. IX.) The rich grays and blues 
and purples of the spherulites and matrix render this a conspicuous 
rock. Other specimens from the same locality (the south flank of the 
mountain northeast of the junction of Copper Pain and Toms Creek) 
show spherulites elongated by flow. They are thus drawn out into 
solid cylinders with a diameter of l mm and a length of some 2 em . 

Specimens of aporhyolites composed of spherulites about the size 
and shape of almonds (18 mni by 9 mm ) were also found. The rock had 
been greatly sheared and the spherulites flattened. 

Spherulites become a still more striking feature of the aporhyolites 
when arranged in layers which traverse the face of the rock in long, 
parallel, dotted bands. This arrangement has been described by 
Iddings in the obsidian of the Yellowstone National Park. 1 

While sometimes these bands are 4 mm wide, at a nearly uniform dis- 
tance apart, and of an indefinite length, in other cases they are very 
narrow, dwindling into mere lines and dying out, to be succeeded by 
other lenticular bands. (PI. X.) The planes of these spherulites have 
become planes of weakness and solution. The rock cleaves readily 
parallel to them and shows a coating of secondary silica on the cleav- 
age surfaces. This deposition of silica causes these planes to become 
the hardest part of the rock, and hence they often stand out as 
parallel ridges on the weathered surfaces. 

At the locality mentioned on pages 41-42, piedmontite, in radiating 
needles, associated rarely with scheelite, has, together with quartz, been 

'Obsidian Clin': Seventh Ann. Rept. U. S. Geol. Survey, 1888, p. 276, PL XVIII. 


deposited along these planes of former spherulitic crystallization. The 
silica occupies the center of the deposit and gives rise to ridged surfaces. 
These layers of sphernlites do not always lie in a single plane. Cross 
sections of the layers show irregular and minute sinuosities, which were 
doubtless caused by movement in the lava during consolidation. 

The bands consist always, when not composed of piedmontite, of a 
central dark line (impurities) with a lighter band (opaque quartz) on 
either side. These in turn are bordered by dark lines (iron oxide) 
(PI. X). This parallel banding, conspicuous even at a considerable dis- 
tance, though recognizable as a true igneous structure by one familiar 
with the acid lavas of the Yellowstone National Park or of the Lipari 
Islands, counterfeits a sedimentary structure so closely that it is not 
surprising that the rocks should be described as "bedded orthofel- 

Still other specimens show spherulites with a tendency to an arrange- 
ment in rows and chains which are in turn collected in bands some 
2 inches in width traversing the rock and alternating with bands 
approximately free from spherulites. This arrangement differs from the 
layer spherulites in the fact that the outline of individual spherulites is 
quite distinct and the grouping irregular. 

Some fine examples of the lithophysal structure were found in the 
Raccoon Creek region (PI. XI). Nothing that could be definitely recog- 
nized as lithophysa3 was observed in the Monterey district. 

In the areas southeast of Jacks Mountain the aporhyolites have 
been more or less silicified or epidotized, or both, and are sometimes 
very difficult to distinguish, in the hand specimen, from the neighbor- 
ing sandstone. A good example of this rock is seen in the exposures 
on the turnpike about one-fourth of a mile above the Emmitsburg toll- 
gate, where silicification has produced a close resemblance to a quartz- 
ite. The fresh surface is white. Feldspar phenocrysts are sparsely 
distributed and scarcely discernable. Quartz blebs are more numer- 
ous. Pyrite is finely disseminated, and the exposed surfaces of the 
rock are tinged with yellow and red iron oxide. The rock cleaves in 
two directions oblique to each other. In this respect it resembles 
many of the felsites. 

The microscopic evidence of igneous character is supported by the 
field evidence, which shows a continuity between this altered rock and 
that which would be at once recognized as a felsite. 



Feldspar and quartz. — The feldspar phenocrysts are very like the 
feldspars of the quartz-porphyries, and little needs to be added to the 
description of them already given. They are fresh, contain inclusions 
of a once glassy magma, show perthitic intergrowth, and are twinned in 
accordance with the Carlsbad (albite) and Manebacher (pericline) laws. 




The specimen is light pink showing on a smooth, weathered surface delicate and sinuous lines of an original fiov 

structure, in shades of red and purple. 

bascom] APORHYOLITES. 45 

In the former case the twinning - is sometimes repeated, and furnishes 
another indication of the triclinic character of the feldspar. 

The feldspars are frequently bent or broken, causing the twinning 
striations to show curvature and faulting. Occasionally a phenocryst 
has been completely broken, the fragments pulled apart, and the space 
filled with a quartz-albite mosaic of much coarser grain than the ground- 
mass, and evidently of subsequent formation. (PL XVII, a.) A com- 
plete replacement of the feldspar crystal by a quartz mosaic sometimes 
occurs. 1 

The quartz phenocrysts have rounded outlines and characteristic 
embayments and inclusions; undulatory extinction is almost universal; 
frequently the quartzes are cracked, and sometimes completely granu- 
lated. Both the porphyritical constituents of the aporhyolites give 
rise, in connection with curving now lines, to the " augen " structure 
which has often been described. 2 

These flow lines leave clear, triangular spaces on one side of the 
phenocrysts, such as Futterer describes.' 

Other porphyritic constituents. — The only ferromagnesian silicate 
unmistakably present in the aporhyolites is biotite. This was found 
in sections representing three different areas of acid volcanics which lie 
along the southern limit of the Monterey district. The first area is a 
small one at the head of Minie Branch. The second area is larger and 
caps the mountain southeast of Raven Rock Mountain. The third 
area is directly to the east of this, on Friends Greek. The lavas of 
these three localities show a marked similarity, both in the hand speci- 
mens and in the thin sections. They are compact, dirty-gray rocks, 
weathering a yellowish-gray. Feldspar phenocrysts are somewhat 
sparsely and inconspicuously present. On a fresh surface biotite can 
be detected with the naked eye. Magnetite is present, and in one of 
two specimens from the mountain locality pyrite is abundant in minute 

All of the thin sections of these rocks are distinguished by finely 
striated feldspars. The carlsbad and x^ericline twinning are sometimes 
both present in a single crystal. Quartz phenocrysts are not numerous. 

The presence of titanic iron oxide in abundance is attested by the 
formation of leucoxene. The groundmass is holocrystalline and finely 
microgranitic, with a tendency toward the micropoikilitic structure. 
This tendency is fully developed in one of the sections. The hand srjeci- 
men, from which this section was cut, shows a rock more completely 
silicified than the other specimens indicate, and also contains pyrite, 
which is indicative of secondary crystallization. 

•Similar replacements have been described by Dr. Milch, Beitnige zur Kenntnis des Verruca no, 
1892, p. 126, 

S J. Lehmann, Untersuehungcn uber die Entstehung deraltkrystallinischen Schiefergeateine, Bonn, 
1884. G. II. Williams, Bull U. S. Geoi. Survey No. G2, pp. 85, 118^ 207, PI. XV, fig. 1. 

3 Karl Futterer, Der " Ganggranit von Grosaaohsen und dor Quartz-porphyr von Thai nn Thiiriuger 
Wald, Iuaug. disser., 1890, p. 32, 


There is some epidotizatioii of the groundmass, resulting in the pro- 
duction of finely granular epidote concentrated at certain centers of 

The mica by which these lavas are characterized has a fibrous char- 
acter, and is the andesitic type described by Koscnbusch. 1 It is in 
ldiomorphic clusters of reddish-yellow fibers. The absorption is char- 
acteristically strong and the pleochroism marked. Parallel to the 
cleavage the fibers show a deep reddish brown color, and at right 
angles a light yellowish-red or straw-yellow color. The marked resem- 
blance of the lava from these localities suggests a continuity in the 
lava liow — a continuity probably maintained underneath the greenstone. 

If this is the case these areas are exposed by erosion of the overly- 
ing basic lava, if it ever completely overlay the aporhyolite. That the 
continuity was maintained above the greenstone is a scarcely tenable 
hypothesis, because of the extended erosion which such a superposition 
would necessitate. 

If any other ferromagnesian constituents were present in the apor- 
hyolites — and there are indications that there were — they have been 
totally decomposed and removed. 

Harker 2 found it to be true of the pre Cambrian acid volcanics of 
Wales, that clusters of biotite Hakes were preserved in a compara- 
tively fresh condition, while only slight trace of augite or hornblende 

In the South Mountain aporhyolites these minerals must have been 
rare originally, and are now completely replaced by epidote. Occa- 
sionally the outline of a perfect crystal section parallel to 010 is pre- 
served, while the substance of the crystal is entirely replaced by epi- 
dote individuals. In the absence of basal sections there is no clue to 
the specific character of the original crystal. 


The groundmass is always a quartz-feldspar aggregate, of varying 
structure and grain (about - 8 \ of a millimeter in diameter), more often 
finely than coarsely microgranitic. The structures of the groundmass 
are of the highest interest and importance in their disclosure of the 
original character of the rocks which possess them. 

The fluidal, micropoikilitic, spherulitic, axiolitic, lithophysal, rhyo- 
litic, micropegmatitic, perlitic,taxitic, amygdaloidal, and trichitic struc- 
tures are characteristically developed and merit detailed description. 

Fluidal structure. — The fluidal structure, Avhich is so familiar to all 
students of rhyohtic lavas, is a conspicuous feature of the aporhyolites, 
both macroscopically, as has been described, and microscopically. 
Globulites of magnetite and hematite and indefinable opaque crystal- 
lites follow sinuous lines of flow, twisting around the pheuocrysts and 

1 Rosenbusch, Massive Gesteine, 2d ed., Vol. II, p. C58. 

2 Bala Volcauic Series of Rocks, p. 18. 



Seven-tenths natural size. 

bascom.] APORHYOLITES. 47 

imparting to tliem the appearance of eyes, previously mentioned. PI. 
XVIII, a, is the reproduction of a photomicrograph of a section show- 
ing this structure. 

Mieropoilcilitic structure. — The micropoikilitic structure has been 
defined in connection with the quartz-porphyries, where it was not 
infrequently a significant feature of the groundinass. It is still more 
characteristic of the aporhyolites, and is occasionally present in the 
basic eruptives. 

These irregular quartz patches, inclosing microlites of lath-shaped 
feldspars or other minerals of independent orientation, give a pro- 
nounced mottled or " patchy" appearance to the groundmass, an 
appearance which has been noted in volcanics of all ages. It has been 
observed and variously described, usually without being named, in 
quartz-porphyries, felsites, porphyrites, rhyolites, and peridotites, by 
numerous writers — Irving, 1 Williams, 2 Haworth,' Cross, 4 Iddings, 5 
Diller, 6 Lindgren, 7 Teall, 8 Harker, Brogger, 10 and Nordensk jold. 1 ' Fel- 
site from the Archean rocks of Georgia shows the same structure. I2 This 
structure is likewise present in the felsites from the neighborhood of 
Boston," as is the case also with the quartz-porphyries and felsites of 
Marblehead Neck, Massachusetts. 

An acid lava of the Keweenawan series from Minnesota, recently 
examined by the writer, shows with a high power and polarized light 
the micropoikilitic structure in perfection. 14 

While the term micropoikilitic is not restricted to a quartz -feldspar 

1 It. D. Irving, Copper bearing rocks of Lake Superior: Mon. U. S. Geol. Survey, Vol. V, 1883, pp. 
99-100, PI. XIII, figs. 13 and 14. 

*G. H. Williams, Neues Jabrbuch fur Min. u ret., Supp. Vol. II, 1882, p. 607, PI. XII, fig. 3. The 
peridotites of the Cortlandt series: Am. Jour. Sci., Vol. XXX, p. 30; Vol. XXXIII, p. 139. 

3 E. Haworth, A contribution to the Archaean geology of Missouri: Am. Geologist, 1888, Vol. I, p. 
3C8, PI. I, figs. 1 and 2; also, Crystalline rocks of Missouri: Ann. Kept. Mo. Geol. Survey, Vol. VIII, 
1894, p. 195, PI. XXII. 

4 Whitman Cross, On some eruptive rocks from Custer County, Colo. : Proc. Colo. Sci. Soc, Vol. II, 
1888, pp. 232, 242. On a series of peculiar schists near Salida, Colo. : Proc. Colo. Sci. Soc, Jan., 1893, p. 8. 

6 J. P. IddingS, The eruptive rocks of Electric Peak and Sepulchre Mountain, Yellowstone National 
Park : Twelfth Ann. Rept. U. S. Geol. Survey, pp. 589, 646. 

C J. S. Oilier, Mica-peridotite from Kentucky: Am. Jour. Sci., 3d series, Vol. XLIV, Oct., 1892, p. 

7 Waldemar Lindgren, On sodalite syenite and other rocks from Montana : Am. Jour. Sci., 3d series, 
Vol. XLV, Apr , 1893, p. 287. 

8 J. J. Harris Teall, British Petrography, 1888, p. 337. 

9 Alfred Harker, Bala Volcanic Series of Rooks, pp. 23, 53, 54. 

10 W. C. Brogger, Der Miueralien der Syenitpcgmatitgangeder siidnorwegischen Augit und Nephe- 
linsyenit: Groth's Zeitschrift fur Krystallographie, Vol. XVI, p. 46. 

"Otto Nbrdenskjold, Zur Kenntniss der sogen. Hallefiinta des nordostliehen Smalands: Bull. Geol. 
Inst. Upsala, Vol. I, No. 1, 1893. 

12 A section of this felsite was loaned by Prof. L. V. Pirsson. It is of especial interest in its 
great similarity to the South Mountain felsite, thereby showing the southward persistence of this 
rock type. 

K 'J. S. Diller, Felsites and assoc. rocks north of Boston: Proc. Boston Soc. Nat. Hist., Vol. XX 
Jan. 21, 1880, pp. 355-368; also Bull. Mus. Comp. Zool. Harvard Coll., whole series, Vol. VII; geol., 
scries, Vol. I. Thin sections of these felsites were kindly loaned by Mr. Diller to the Johns Hopkins 
laboratory. They have many microscopic features in common with the South Mountain rocks, and 
like them were first supposed to bo of sedimentary origin. 

l4 Tlie writer's attention was called to the presence of this structure in the Minnesota acid volcanics 
by Dr. U. S. Grant, of the Minnesota State Geoloyical Survey. 


intergrowth, in most of the occurrences described these have been the 
component minerals. In the South Mountain rocks the feldspathic 
material is usually so abundant as not to admit of the determination 
of the mineral character of the host. In such cases a clue to the 
nature of the cementing material is found in its optical continuity with 
the porphyritic quartz. These phenocrysts are severally included 
within a single micropoikilitic area, with which they are always simi- 
larly oriented (PL XVH, b). The cementing material acts as a sort of 
secondary enlargement of the quartz phenocrysts. The feldspar phen- 
ocrysts, on the other hand, have no effect upon the orientation of the 

In the basic rocks, which are coarser-grained, the character of the 
host can be directly tested and proved to be quartz. The mottled 
appearance, previously alluded to, is usually emphasized by the 
arrangement of globulites, longulites, and trichites of iron oxide. This 
is such as both to accord with the flow structure and to outline the 
quartz areas, which in these cases have a somewhat oval shape. 

When shearing has led to the production of sericite, this mineral is 
formed around the micropoikilitic areas, rarely traversing a single 
area, when it seems to be cementing material filling a crack (PL XVII, 
b). These areas are persistent, and slowly disappear in the develop- 
ment of a slate. 

While in some cases this structure is undoubtedly of primary char- 
acter, as Professor Iddings considers it to be in many porphyrites, in a 
large class of rocks its secondary origin seems equally certain. Irving, 
who gives one of the best as well as the earliest (1881) descriptions of 
this structure, considered it of a secondary character. His statements 
as to its nature and origin are so applicable to the South Mountain 
aporhyolites that they are quoted here in full: 1 

Although wholly absent from some sections, a very highly characteristic feature of 
the sections of many of these rocks, and more particularly of the fclsites without 
porphyritic quartz, is a network quartz which can only be regarded as of secondary 
origin. I find no mention of such a feature in any of the descriptions of the felsites 
of other regions which I have examined. Only occasionally is this net- 

work quartz coarse enough to be readily seen with a low power in the ordinary 
light. Usually both a high power and the use of the polarized light are required 
for its detection, when it appears in its most characteristic development as a deli- 
cate aborescent tracery or frost-work saturating the ground mass in all directions. 

In the polarized light all of the quartz network within each of these numberless 
irregularly round areas, whose existence would not be suspected in the ordinary 
light, is found to be similarly oriented. 

From these more pronounced developments the secondary quartz is found through 
many degrees of lessening amount and less plainly marked character until it disap- 
pears altogether. It is plainly of the same nature as the secondary quartz of the 
already described orthoclase-gabbro, diabase-porphyrite, and quartzless porphyry, 
and of the augite-syenite described below. It never, however, reaches in the rocks 
now under description the coarseness nor presents the graphic forms with which it 
appears in the augite-syenites, its characteristic development here being the deli- 

^oc. cit., pp. 99-100. 


JLLETIN NO. 136 P,_. X 

'4 ' " ?V ^W^~\- 


_j — s 


From Jim Saunder's farm, Bigham Copper mine district. Five-sevenths natuial size. The details of the 
spherulitic chains are lost in the photograph. 

bascom.] APORHYOLITES. 49 

cate arborescent clusters above mentioned. Whether this secondary quartz may ever 
be rather a result of devitrification than a truly secondary or alteration product, I 
have no means of deciding, though it is certainly the latter often, and I should sup- 
pose always. It surely can have no connection with the original solidification of the 

Observations made on the South Mountain aporhyolites lead to 
essentially the same conclusions as those reached by Irving. As the 
nature of the structure is of both interest and importance in its bear- 
ing upon the question of the primary or secondary character of the 
groundmass, attention will be called to some of the observations which 
prove suggestive. 

It has been stated that a few sections of the basic lavas of South 
Mountain exhibited this structure. In these sections the nature of the 
structure could be more readily detected. (PI. XIX, a and b.) 

The outline of the lath-shaped feldspars, forming an original ophitic 
structure, is completely preserved, though none of the original constit- 
uents of the rock remain, unless some of the titaniferous iron oxide is 

At present the rock consists entirely of cpiartz, epidote, magnetite 
(or ilinenite), aud leucoxene. It is amygdaloidal, and the vesicles are 
filled with secondary quartz. Quartz is also a cement for the minerals 
of the groundmass, and forms irregular interlocking areas, which are 
quite similar to the micropoikihtic areas of the finer-grained acid rocks, 
and produce in polarized light the familiar patchy effect. Fine cracks 
traversing the sections are still preserved in outline by the ferrite, but 
are prior to the quartz areas, which have obliterated all trace of cement- 
ing material. The epidote, which replaces crystals of some former ferro- 
magnesian constituent, is often pierced by these cracks, which become 
invisible in the quartz areas save for the outlining ferrite. There can 
be no question as to the secondary character of the micropoikihtic 
structure in these cases. 

Some structures described by Zirkel l in the rhyolites of the Washoe 
district are suggestive in this connection. He notes (slide 350, fig. 1, 
PI. VI) perlitic parting in certain rhyolites (southeast from Wadsworth), 
where the cracks, semicircular and oval, traverse a glassy groundmass 
and are bordered by a narrow zone of microfelsite, "giving [to the sec- 
tion] the appearance of a network." The same general effect is produced 
in other rhyolites of the district (sees. 351, 352, figs. 2, 3, PL VI) by 
faint granular lines ''which, by their fluidal running, form a net with 
a multitude of meshes of oval shape." These lines seemed to be the 
vestiges of perlitic cracks, though they could not be certainly deter- 
mined as such. A widespread and characteristic type of rhyolite (sees. 
333, 407, fig. 1, PI. VIII) shows the same network of dark granular lines, 
but in this case the meshes are filled with a spherulitic crystallization. 

There are, then, two types of crystallization which may occupy the 

1 Geol. Expl., 40th parallel, Vol. VI, Microscopic Petrography. 

Bull. 13G 4 


oval spaces, the spherulitic and the microfelsitic. The former does so to 
the exclusion of a glassy groundmass, and must have been formed prior 
to the consolidation of the rock. The latter shares the space with the 
glass into which it gradually passes and of which it may be a molecular 
alteration. Turning to the South Mountain aporhyolites (particularly 
the spherutaxites), we find a similarity in the development of faint gran- 
ular lines forming irregular oval meshes and giving to the section the 
network appearance figured by Zirkel. These lines maybe traces of a 
former perlitic parting or of a microrluidal structure. The meshes are 
now filled either by the quartz areas which condition the micropoiki- 
litic structure or by the vestiges of a spherulitic crystallization. 

The latter represents a primary structure, as in the rhyolites; the 
former may represent the molecular rearrangement of a spherulitic 
crystallization or of a glassy groundmass. In the last case it is the 
direct result ot devitrification and infiltration, processes more readily 
initiated along the borders of the perlitic cracks, as in the rhyolites of 
the Washoe district. 

A comparative study of some sections 1 of the rhyolites of the Rosita 
Hills, Colorado, and of Obsidian Cliff, Yellowstone National Park, 
side by side with the aporhyolites under discussion, also suggests the 
secondary character of the micropoikilitic structure with reference to 
the spherulitic crystallization. In the trichitic spherulites of the mod- 
ern rhyolites there is an appearance analogous to the micropoikilitic 
mottling, caused by the breaking up of the radiating spherulitic fibers 
into irregular areas which extinguish differently. An altogether simi- 
lar phenomenon occurs in some of the spherulites of the ancient rhyo- 
lites. It is indicative of an intermediate stage between the spherulitic 
and a completely micropoikilitic crystallization. This change from the 
spherulitic to the micropoikilitic structure is carried still further in 
some sections, notably in the case of a specimen crowded with minute 
red spherulites. Each spherulite extinguishes as an individual filled 
with inclusions of feldspar and iron oxide (hematite). The host can be 
determined by optical tests to be quartz. The shape of the spherical 
bodies in the hand specimen and in thin section, their tendency to form 
bands and chains, and their uniform color put their original spherulitic 
character and the secondary nature of the micropoikilitic structure 
beyond doubt. It is not supposed that a prior spherulitic crystalliza- 
tion always existed where now the aporhyolites show a micropoilitic 
structure, but these evidences of the derivation of the structure from 
spherulites establish a presumption for its secondary origin in other 
aporhyolites, where it may be the direct result of devitrification or 
may be due to the subsequent alteration, by infiltration, of a granular 

On the whole, the plainly secondary character of the micropoikilitic 

1 Sections of material from these localities wore kindly loaned the writer by Dr. Cross and Professor 

bascom.] APORHYOLITES. 51 

structure in the basic volcanic, the evidence in the aporhyolites of its 
being subsequent to fluidal lines or to perlitic parting, the indications 
that in many cases it is subsequent to a spherulitic crystallization, all 
denote a secondary origin for this structure in the South Mountain rocks. 

Spherulitic structure. — There are two sorts of spherulitic crystalliza- 
tion in the aporhyolites. They differ in no essential respect, but are 
unlike in appearance. The most numerous spherulites are also the 
simplest and smallest. They arc colorless, microscopic spheres, scarcely 
or not at all perceptible in ordinary light, but between crossed nicols 
showing a distinct dark cross. Spherulites in every respect similar 
have been described and figured by Professor Iddings from the rhyo- 
lites of the Yellowstone National Park. 1 Similar spherulites also occur 
in the rhyolites of Hungary and in the felsites of the Boston Basin. 
These radial growths are grouped in bunches and along lines, and are 
composed of positive fibers. Further optical determination of the fibers 
could not be made. Their positive character indicates either a prismatic 
section of quartz elongated in the direction of the vertical axis or a 
clinopinocoidal (010) section of orthoclase elongated in the same direc- 
tion. In the latter case the extinction would be slightly inclined. 
This is impossible, however, of determination. 

While it is not impossible that some of these spherulites are second- 
ary, in some cases there is evidence of their primary character. One 
such case of spherulites whose formation was coincident with the con- 
solidation of the rock occurs in an aporhyolite from a cut made by the 
Gettysburg Railroad north of Toms Creek trestle. Minute colorless 
spherulites are embedded in a base which suggests in every way its 
former glassy condition. In ordinary light there is no evidence of 
crystallization except the porphyritical. 

The groundinass is traversed by irregular, angular cracks, evidently 
the result of crushing. These cracks, which are cemented by epidote, 
pass through the spherulitic aggregates (seen with crossed nicols), some- 
times cutting directly across a spherulite, portions of which appear on 
either side of the crack. Between crossed nicols the field breaks up 
into a mosaic of quartz and feldspar. The granular crystallization 
disregards the cracks, filling the spaces left by the cementing epidote. 
It seems fair to conclude from these observations that the spherulitic 
crystallization is prior to the cracking, that the granular crystallization 
is subsequent, and that the cracking occurred in an already solidified 

The secondary character of the granular crystallization admitted, it 
is easy to suppose that it is due to devitrification continued through 
lapse of time. The spherulites, on the other hand, being prior to 
the crystallization of the groundmass, and prior to the cracking, are 
doubtless primary and contemporaneous with the consolidation of the 

1 Obsidian Cliff': Seventh Ann. Rept. U. S. Geol. Survey, Pi. XVII, p. '276. 


The other class of spherulites correspond to those figured by Professor 
Iddings. 1 They are much larger than those that have just been 
described. The smallest is easily discernible by the naked eye, while 
the largest is 12J cm. in diameter. They are spherical, hemispher- 
ical, cylindrical, fan-shaped, oval, or irregular in form. While they 
all possess a clear-cut, conspicuous outline in ordinary light, in many 
sections they completely disappear between crossed nicols. The fresh 
spherulites (PI. XXII, 6), which still show in polarized light a radi- 
ating structure, are usually colored red by a finely disseminated iron 
oxide. The globulites of hematite are distributed homogeneously 
throughout the spherulite, or they are grouped in radial and concentric 
lines. These lines are most dense near the margin of the spherulite, 
and may be separated from the central portion by a narrow clear zone, or 
it is the outer rim of the spherulite which is clear. Iu the central por- 
tion of the spherulite the coloring matter shows a tendency to collect 
in bunches that correspond with areas which extinguish as individuals. 
Between crossed nicols the field is broken up into the minute areas 
which were referred to on page 50 as forming a structure approaching 
the micropoikilitic. Feldspar phenocrysts usually occupy the center of 
the radiating crystallization, and two or more spherulitic centers may 
be included within a single outer zone. In specimens from Raccoon 
Creek 2 these radial growths were remarkably well preserved and occur 
in a groundmass which retains the characteristics of a glass in great 
perfection (PI. XX, a; PI. XXI, a). It bears a close resemblance to 
the groundmass of some of the Colorado rhyolites, and in ordinary light 
would certainly be mistaken for the base of a fresh glassy lava. Deli- 
cate perlitic parting, Avhich because of its delicacy is easily obliterated, 
is here preserved in wonderful detail. It is evidently subsequent to 
the radial crystallization to which it accommodates itself. The pres- 
ence of innumerable globulites accentuates the perlitic and rhyolitic 
structures. With crossed nicols the groundmass at once betrays its 
noncrystalline character (PI. XX, b). All glassy structures disappear, 
to be replaced by granular quartz and feldspar, a crystallization which 
closes the perlitic parting and thereby completely obliterates it. 

The porphyritic feldspars still show inclusions of a glassy base. It is 
impossible by any description to produce that definiteness of conviction 
as to the original glassy nature of the groundmass which the character 
of the sections justifies. To one who has studied these sections in both 
ordinary and polarized light there can be no question as to the secondary 
character of the holocrystalline groundmass. One can not escape the 
conviction that the rock originally consolidated as a spherulitic perlite 
and has become holocrystalline by a process of devitrification. 

The sequence of events, as revealed by microscopic study, is as fol- 
ows : There was first the intratelluric development of the porphyritic 

1 Op. cit., p. 277, PI. XVII. 

2 In Franklin County, Pa., east of Kocky liidge and south of Graeffenburg. 

bascom] APORHYOLITES. 53 

crystals, followed by tlie emergence of the magina, the development of 
globulites and fluxion structure, the commencement of radial crystalli- 
zation, and finally the consolidation of the magma as a glass and the 
development of perlitic parting. Subsequent to all this, and extending 
over a much longer period of time, the process of devitrification took 

Associated with a holocrystalline groundmass, which bears less 
marked evidence of an original glassy character, are the more altered 
spherulites. Their circular outline in the thin section and their spher- 
ical shape in the hand specimen testify to their former presence. 

In both hand specimen and thin section a threefold zonal arrangement 
is often clearly defined by the distribution of red or black iron oxide. 
With crossed nicols these boundaries become inconspicuous, and the 
field of the microscope shows only a uniform quartz-feldspar mosaic, 
or the former radial growth is indicated by zones of a finer-grained 
crystallization than the groundmass (PI. XXIII, a), or a micropoikilitic 
structure is present within the spherulitic boundaries when absent in 
the groundmass. Occasionally, vestiges of a radiate structure still 

In the specimen already referred to on page 50 and figured in PI. 
XXII, a, the spherulites are colored red by a uniform dissemination of 
hematite particles, and are not more than one-half mm. in diameter. 
They traverse the rock in rows and chains, which are in turn grouped in 
bands about 2 inches wide. The rock has been sheared, a ready cleav- 
; age has been produced, and sericite has developed around each spher- 
I nlite. As has already been mentioned, the spherulitic individuals have 
t become micropoikilitic individuals. Another specimen shows spheru- 
lites which have been rendered almond-shaped through shearing or, 
less probably, by the fluidal movement of the magma during this 
I consolidation. That the former was probably the case is indicated by 
the gradual passage of the rock into a slate and the development of 
Spherulites which, like these, have been replaced by a fine-grained 
! mosaic of quartz and feldspar have been described by Klockmann. 1 
Chain spherulites. — The arrangement of spherulites in layers and along 
planes so that a cross-section shows a chain of spherulites has been 
j described on page 43 and figured on PI. XVIII, b, as it appears in the 
I hand specimen. In ordinary light the microscope discloses somewhat 
sinuous or straight dark bands, with scalloped borders sharply outlined, 
inclosing an irregular clear chain which also has scalloped edges. 

Frequently there are detached clear spots with circular outline. 
These bands sometimes spread out so as to include phenocrysts and 
sometimes curve around them. (PI. XVIII, b.) A comparative study of 
these spherulitic chains and the chains of fresh spherulite of the Yel- 

' Die Torpbyre ; Der geologisobo Aufbau des sogen. Magdebnrger Uferrandes niit besonderer Berttck- 
sicbtiguiig der auftreteudeu Eruptivgesteine: Jabrbucb K. prrnss. geol. Landesaustalt, Vol. XI, 
1890. p. 179. 


lowstone obsidians discloses a striking similarity in ordinary light — 
the same irregularly scalloped outline, the same central chain of clear 
spherules. With crossed nicols the close similarity vanishes, for in the 
ancient rocks the radial growth has utterly disappeared. The central 
clear chain consists now of a fine quartz mosaic. The dark borders, 
except- for the crowded magnetite globulites, can not be distinguished 
from the noncrystalline quartz feldspar groundmass. This clear cen- 
tral zone where the spherulites converged evidently furnished the plane 
of weakness and easy solution, along which silica was infiltrated and 
parallel to which the rock cleaves. 

The impurities which have entered the rock along this cleavage plana 
give rise to the central dark line mentioned in the macroscopic descrip- 
tion, while the silica forms the opaque white band on either side. The 
central zone is sometimes more than a millimeter wide. Where a feld- 
spar crystal lies across this plane of weakness with its longest axis at 
right angles to the latter, the strain has proved too great for the crystal, 
which has been broken apart and the break cemented by infiltrated 
silica (PI. XXIII, h). 

The chain spherulite structure is of more common occurrence in the 
aporhyolites of the Monterey district than any other form of spheruj 
litic growth. The acid rocks east of the Bigham copper mine show 
them in great perfection. llutley 1 has figured some similar chain 
spherulites in the felsitic lavas of England and Wales. In felsite of 
the Keweenaw series from the Minnesota shore of Lake Superior the 
writer has recently observed fine bands of silica so similar to the altered 
chain spherulites as to suggest a like explanation for them. 

Axiolitic structure. — Closely related genetically to the chain spheru- 
lites, but unlike them in being radial linearly rather than centrally, is 
the axiolitic growth. 2 

Axiolites are not particularly characteristic of the South Mountain 
aporphyolites. Curving, linearly radiating growths do occur, however, 
in specimens from more than one locality. PI. XXI, Z>, shows this 

Rhyolitic structure. — The rocks in which the axiolites were observed 
arc noncrystalline, yet they exhibit most strikingly the characteristics 
of a glass. Flow and vesicular structures, stringers and shreds, and 
curved patches of a brownish-red color, forming what has been called 
the rhyolitic structure, abound. (PI. XXIV, a and b; PI. XXV, a.) 
This latter structure has been figured and described by Rutley, 3 Xor- 
denskjold, 4 and de la Vallee-Poussin, 5 and on a macroscopic scale by 

felsitic lavas of England and Wales: Mem. Geol. Survey, Gt. Brit., 1885, Pi. VII, figs. 11 and 12. 

: Zirkel, Microscopic Petrography : Geol. Expl. 40th parallel, p. 1G7. 

3 Rutley, On the microscopic structure of devitrified rocks from Beddgelert and Snowden: Quart. 
Jour. Geol. Soc. London, Vol. XXXVII, p. 406, figs. 1 and 2. 

'Xordenskjold, Zur Kenntniss des sogen. Halleflinta des nordbstlichen Smalands: Bull. Geol. Inst. 
Upsala, Xo. 1, Vol. I, p. 5, 1893. 

f Dela Vallee-Poussin, Les anciennes rhyolites, dites eurites, deGrand-Manil: Bull. Acad. Roy. Bel- 
gique, 3d series, Vol. X, 1885, p. 271. 









* at* 

V* «,"•■ 

% v 



The rock is a pale pink, while the originally hollow spaces between the concentric layers of the lithophysae are 
filled with a deeper pink siliceous deposit. 

bascom.] APORHYOLITES. 55 

Irving.' A perlite from Deer Creek Meadows, 25 miles south of Lassen 
Peak, displays a similar rliyolitic structure. This structure is essen- 
tially a phase of the fluidal structure. 2 

Lithophysal structure. — Very often the macroscopic features of the 
aporhyolites disclose their original character more convincingly than do 
the microscopic. Lithophysal are best revealed in the hand specimen? 
where they are brought out in delicate relief by weathering. In such 
specimens from the Raccoon Creek locality, the rose-pink petals of the 
lithophysa' in a pale pink base produce quite as beautiful examples of 
this glassy structure as any obsidian or rhyolite offers. (PL XI.) No 
undoubted lithophysal were found within the Monterey district. The 
microscope discloses some vesicular structures which bear slight trace 
of a lithophysal character, but the alteration has been too great to 
allow of their identification as hollow spherulites. 

Mieropcgmatitic structure. — Themicropeginatitic structure shows itself 
in microscopic pegmatoid groups of phenocrysts, such as have been 
frecpiently described in porphyries and rhyolites. 5 It does not play an 
important part in the aporhyolites. 

Perlitic structure. — That this structure is present in the South Moun- 
tain rocks, and in great perfection, has already been noted. (PI. XX, a, 
PI. XXI, a.) While its presence is a most reliable proof of the former 
character of the rock, its absence furnishes no evidence against the pre- 
vious glassy condition of the rock, both because many recent rhyolites 
showed no trace of that structure and because it is most readily effaced 
by devitrification. 

Amygdaloidal structure. — At Paccoon Creek, at the Bigham copper 
mine and its near vicinity, are light-colored (pink and yellow), extremely 
vesicular aporhyolites. The vesicles are oval or elongated by How move- 
ment. (PI. XXVI, a and b.) They are uniformly filled with epidote or 
quartz, with both, or with either to the exclusion of the other. When 
both are present the quartz forms a rim around the epidote. The epidote 
has often a radial arrangement, while crystal boundaries are absent. Its 
color varies from a deep yellowish green to light yellow, and pleochro- 
ism is marked. In some of the larger amygdules the radiating needles 
of epidote have been broken and pulled apart at right angles to their 
longer axis and the spaces filled with silica. Piedmontite and quartz 
show the same relation, as described on page 41. The groundwork of 
these amygdaloids is the usual holocrystalline quartz-feldspar aggre- 
gate. Incipient alteration to granular epidote is more frequent in these 
open- textured amygdaloids than in the compact aporhyolites. These 

■Irving, Copper-bearing rocks, etc.: Mon. U. S. Geol. Survey, Vol. V, pp. 312-313, fig. 22. 

2 Thisisthe " Aschenstructur " of Miigge (Untersuehungen fiber dio "Lenneporphyro'' in Westfalen 
und den angrenzenden Gebieten: Neues Jalirb. fur Min., Geol. u. Pal., B. B. VIII, 1S93, pp. 048, G19, 
713), who considers ifc due to the original fragmental character of the lava. Whether it is always to 
be so interpreted is a question for further investigation. 

1 Iddings, op. cit., p. 275, PI. XV, fig. 5. 


rocks are pierced with coarse quartz veins which, bear native copper. 
Copper in microscopic quantities and copper oxide also occur m the 
amygdules. Hand specimens are frequently coated with the copper 
carbonates, malachite and azurite. There were picked up on the road- 
side some specimens of amygdaloidal aporhyolites that are quite 
diverse from the amygdaloids of the Bigham copper mine which have 
just been described. They are similar to specimens found north of the 
Monterey district at Eaccoon Greek. The amygdules, which are black 
against a yellowish- white background, are finely attenuated and elon- 
gated in long parallel hair lines, lending to the rock the appearance of 
an eutaxite. The black color is due to magnetite, which either is finely 
disseminated in quartz (the other infiltrated mineral), or is present in 
masses, or simply forms a heavy rim around the amygdules. 

Some of the amygdaloids from Eaccoon Creek merit a detailed descrip- 
tion. In these rocks the vesicles are usually bordered by a broad rim 
like the groundmass, in its present crystallization, but separated fruin 
it by a narrow, clear zone of quartz, and characterized by a greater 
abundance of magnetite (or ilmenite). On the inner edge of this 
border are spherulitic growths, while the rest of the vesicle is filled 
with quartz (PI. XXVII, b) or with quartz and an opaque black oxide 
(PI. XXVII, a). In the latter case the black oxide occupies the center 
of the vesicle, leaving a clear zone of silica around the sxmerulites. 
Crossed nicols show that the spherulites are optically continuous with 
the quartz, and that the radial appearance which has been retained is 
due to the arrangement of the impurities. The appearance of these 
vesicles is very suggestive of those figured by Professor Cole. 1 Pro- 
fessor Cole explains this type of spherulite by a dual mode of growth — 
a radial growth outward from the groundmass, as well as inward, origi- 
nating in the glass and converging toward a spherulitic center ! He 
does not give the mineral character of the spherulite. Whatever may 
be the facts with reference to the Eocche Eosse obsidian, it is not 
necessary to postulate an abnormal method of crystallization to explain 
the phenomena observed in the South Mountain aporhyolites. 

The spherulites projecting into the vesicle, with their bases sunk into 
its walls, were recognized by Professor Iddings, who kindly examined 
the section, as tridymite spherulites, such as form on the walls of vesic- 
ular cavities in all kinds of modern lavas. Tangential sections of such 
spherulites are represented by granular aggregations. The form of 
the tridymite has been preserved by impurities, while its molecular 
arrangement has been altered to that of quartz. The presence of a 
border between the groundmass and the cavity suggests that crystal- 
lization, starting from the walls of the cavity, took place within the 
magma, initated, perhaps, by the gaseous content of the vesicle. 

'Grenville A. J. Cole and Gerard W.Butler, On the lithophyses in the obsidian of the liocche 
Rosse, Lipari : Quart. Jour. Geol. Soc. London, Vol. XLVIII, 1892, p. 438. 

bascom.] APORHYOLITES. 57 

Somewhat similar radial growths within vesicles in ancient rhyolites 
have been described and figured by de la Vallee-Poussin. 1 

Taxitic structure. — Another structure which the South Mountain rocks 
possess in common with rhyolites is what has been called the taxitic. 2 
This consists in the intimate mingling of two portions of the magma 
which from some cause (liquation) are slightly differentiated. The 
iron constituent, which evidently separated out in the original glass, 
has been still further crowded into bands and curved lines by the 
secondary crystallization. The result is the production, in some cases, 
of an irregular mottling, when the rock is called an ataxite; and in 
other cases of a more or less complex network of interlacing bands 
following lines of flow and forming a eutaxite. This mottling and 
banding is made the more striking by a marked contrast in color. The 
body of the rock is light-gray or pink, and the lines are dark blue-gray 
or red, according to the varying degrees of oxidation of the iron. 
Where the dark lines outline oval and spherical spaces and contain 
porphyritical crystals iu or near their centers, the crystallization is 
regarded as having once been spherulitic and the rock is termed a 
spherutaxite. These have been described on page 50. 

The eutaxites are frequently so sheared as to give a hair-like tenuity 
to the bands in cross section, while the microscopic slide shows the 
effect of pressure on the rock in the parallel arrangement of the glob- 
ulites of black oxide. The universal presence of globulites, trichites, 
and microlites of black and red iron oxide in flow bands, or indiffer- 
ently distributed, or in concentric zones around spherulites and vesi- 
cles, is worthy of mention as a further point of resemblance to the mod- 
ern rhyolite. Such a trichitic structure in similar rocks has been 
described by various petrographers. 3 


It is not easy to present the evidence for the secondary nature 
of the holocrystalline groundmass so that it shall have the weight 
which properly belongs to it. Very much depends upon effects which 
it is impossible to convey by description, but which carry conviction 
to the student of these rocks. The contrasting appearance of many of 
the sections in ordinary and polarized light can not be adequately repro- 
duced. The disappearance under crossed nicols of rhyolitic, perlitic, 

'Lesanciennes rhyolites, elites eurites, de Grand-Mauil : Bull. Acad. roy. Belgique, Lid series, Vol. X, 
1885, p. 292. 

"Fritsch and Reisa, Teneriffe, 1808, p. 414. 

Roaenbusch, Mic. Phys. der Massigen Gesteine, 2d ed., p. 625. 

F. Loewinson-Lessing, Zur Bildungsweise uiid Classification der klastischen Gesteine, 1888, pp. 228- 
235. Note sur les taxitea et aur lea roches clastique vulcanici tie : Bull. Soe. Belg. geol., etc., Vol. V., 
1891 Loewinaon-Lessing's division of the taxitea into ataxites, eutaxites, and spherutaxitea lias 
been followed in this bulletin. 

S R. D. Irving, Copper bearing Rocks, etc. : Mon. U. S. Geol. Survey, Vol. V, p. 312. 

S. Allnort, On certain ancient devitrified pitchstones and perlites from the Lower Silurian district 
of Shropshire. Quart. Jour. Geol. Soe. London, Vol. XXXIII, p. 449. 

Nordenskjold, op. eit. 


spherulitic, and fluxion structures, so clearly indicated in ordinary 
light, in a homogeneous noncrystalline mosaic is one of the strongest 
evidences for the secondary character of the crystallization. 

There are also instances where the nature of the crystallization is 
distinctly proved. On page 51 it was shown to be subsequent to the 
cracking which must have occurred in a solid rock. Pages 52-53 
describe the replacement of the radiating crystallization of the spheru- 
lites and chain spherulites by a granular crystallization which is homo- 
geneous with a granular groundmass. Finally, on pages 49-50, the 
secondary character of the micropoikilitic crystallization has been 
indicated. One or another of these indications of secondary crystalli- 
zation is almost invariably present in the rocks which have been 
included under the name aporhyolite. 

The exceptional occurrences, where these structures are absent, show 
genetic relationship in the field to typical aporhyolites. The deter- 
mination of the character of the groundmass in the cases described 
thus practically determines it for all the aporhyolites. 

The secondary character of the holocrystalline groundmass once 
admitted, and the indications of an original glassy base recognized as 
such, one is forced to conclude that the former was developed from the 
latter by a process of devitrification. 

That the processes of crystallization do not necessarily cease with 
the solidification of a magma is well known, for experiment has x^roved 
that crystallizing forces are active in a glass as well as in a molten 
magma. 1 This action is exceedingly sluggish, and requires, unless 
accelerated by heat and moisture, an immense amount of time. Devit- 
rification has been considered the result of dynamic action only; 2 but 
while dynamic action undoubtedly accelerates the process, if it does not 
initiate it, devitrification may also take place independently of dynamic 
action, as was the case in the famous example of the old cathedral win- 
dow glass 3 and the ancient devitrified glass from Nineveh investigated 
by Sir David Brewster. 4 The nature of the process is in no way differ- 
ent from the process of crystallization in a fluid magma save in the 
rapidity of the action, and is of both a physical and a chemical character. 

The devitrification which has occurred in the South Mountain apo- 
rhyolites is not attributed to dynamic action, of which there are many 
evidences of another nature in the South Mountain, but to statical 
metamorphism. The former would, by shearing, obliterate the original 
structures of a glassy rock and produce a slate, while the latter might 
be an important initiatory and accelerating factor in the process of 
devitrification of the glassy rocks. 

'Daubree, Geologie experimental, 1879, p. 158. 

'-De la Vallee Toussin, Les eurites quartzeuses (rhyolites anciennes) <le Xivelles et ties environs: 
Hull. Acad. roy. sci. lett. et des beaux arts <le Belgique, 57 annee, 3d series, Vol. XIII, No. 5, 1887, 
pp. 521-522. T. G. Bornemann, Der Quartzporphyr von Heiligenstein und seine Fluidalstructur: 
Zeitschr. Deutsch. geol. Gesell., Vol. XXXIX, 1887, p. 793. 

3 British Ass. Ptept., 1840. 

"Trans. Royal Soc. Edinburgh, Vols. XXXII, XXXIII. 

bascom.] APORHYOLITES. 59 

Opinions of petrographers. — Paleozoic and pre-Paleozoic acid vol- 
canics have long been studied on the European continent. Although 
their variation from the modern type of acid volcanics, rather than their 
resemblance to that type, has, for the most part, been emphasized by 
German and French petrographers, there have not been wanting able 
advocates of devitrification and of an original glassy base for the 
ancient lavas. 

11. Ludwig' (1861) and Vogelsang 2 (1867) inclined to the opinion that 
the ground mass of certain quartz-porphyries is the result of the devit- 
rification of a glassy lava. 

The late Dr. K. S. Lossen 3 (1869), on comparing the spherulitic por- 
phyries of the Harz Mountains with the obsidians of Lipari, Mexico, and 
Java, found the resemblance sufficiently striking to lead him to declare 
that the porphyry groundmass was originally crystallized as glass, and 
became cryptocrystalliue through molecular rearrangement. Later, 
Kalkowsky 4 (1874) suggested that devitrification through the chem- 
ical activity of water was the process by which the microfelsitic base of 
certain pitchstones and'felsites was developed; and still later, H. Otto 
Lang 5 (1877) described a macroscopically unindividualized base which 
is similar macroscopically to the devitrified base described by Kalkow- 
sky. Sauer ° (1889) considered the Dobritz porphyries as the final altera- 
tion product of a pitchstone. More recently Klockmann 7 (1890) has 
described the replacement of the spherulitic crystallization in quartz 
porphyries, through secondary processes, by a fine-grained aggregate 
of quartz and feldspar. 

Osann 8 (1891) described incipient devitrification in perlite and other 
glassy rocks from Cabo de Gata. Finally, Link (1892) considered that 
the fine-grained groundmass of some American rocks closely related 
to mica-syenite-porphyries was once glassy, or at least partially glassy, 
and 0. Vogel 9 (1892) reached the same conclusion as to the Umstadt 
porphyries in Hessen. Many no less capable observers still hold to 
an original difference between ancient and recent acid volcanics, and 
the possibility of devitrification and an original similarity is yet an 
open question in Germany. 

In France, La Croix 10 describes andesites from Martinique in which 
the glass has altered into quartz spheruhtes and a granular quartz 

It is interesting to note that many of the hallefiinta of Sweden, 

>Erl. zur geol. Karte Hessens, Bl. Dieburg, 1861, p. 56. 
2Phil. do geologie, pp. 144, 153, 194. 

3 Beitrage zur Petrograpbio dor plutoniscben Oeateino: Abhandl. der Berliner Acad., 1869, p. 85. 
4 Tsehennaks mineral. Mittbeil, pp. 31, 58. 
f 'Grundri.s.s der Gesteinskunde, p. 43. 

6 Erl. zur geol. Spocialkarto Sacksens. Bl. Meissen, pp. 81-91. 

'Die Porpbyro; Dor geol. Auf ban sogen. Magdeburger U ferrandes mit besonderer Beriicksichti- 
gung der auftretenden Ernptivgesteine : Jabrbucb K., preuss. geol. Landesanstalt,Vol. XI. 
» Zeitsohr. Deutsch. geol. Gesell., Berlin, pp. 691, 716. 
9 Abhandl. geol. Landesanatalt von llosson, Vol. II, p. 38. 
10 Coruptes-rendus, CXI, p. 71. 


which, like the South Mountain volcanies, were once described as sedi- 
mentary, are proving to be acid volcanies, preserving the features of 
their modern equivalents. Quite recently glassy and rhyohtic struc- 
tures in these rocks Lave been observed and described by Otto Nor- 
denskjold. 1 

In Belgium, de la Vallee-Poussin seems to be the only writer who has 
brought out the resemblance between the eurites of that country and 
modern rhyolites. He describes at some length structures similar to 
those possessed by the aporhyolites of South Mountain. A vacillating 
state of mind as to the matter of nomenclature is indicated in the 
titles of his successive papers. 2 

In England the rhyolitic character of ancient acid volcanies has been 
recognized and emphasized, and the idea of devitrification is widely 
accepted. Allport, Cole, Bonney, Butley, Judd, and Harker have 
accomplished most valuable work along this line. 

Dr. Wadsworth 3 was the first American petrographer to advocate 
the abandonment of age as a factor in rock classification, while at the 
same time he recognized devitrification as the process which was form- 
ing felsites out of rhyolites. What he says is of interest in its anticipa- 
tion of ideas now more generally accepted: 

This devitrification gives rise in the older and more altered rhyolites to the feld- 
spar, quartz, and microfelsitic (so called) hase that has so puzzled lithologists in the 
study of the felsites. The rhyolites of all volcanic rocks preeminently show lamina- 
tion produced by flowing, a fact which is doubtless duo to their being so siliceous. 
This structure and their devitrification enable us to trace a direct connection 
between the rhyolites and felsites, which are simply the older and more altered rhy- 
olites. * * * Q ne f the best illustrations of this is to be found on Marblehead 
Neck, Massachusetts, where at least two distinct flows of felsite occur, one cutting 
the other. They show the fluidal structure so characteristic of rhyolites — a char- 
acter that has been mistaken for lines of sedimentation by geologists, while the 
inclosed crystals of orthoclase have been taken for pebbles. While to the 

naked eye and under the microscope this rock shows the fluidal structure of a rhy- 
olite, iu polarized light it i3 seen that the base has been completely devitrified, a 
process that is carried to a great extent in many known modern rhyolites. 

No other American petrographer has so distinctly advocated the 
identity of felsites and ancient rhyolites, in spite of the fact that many 
of our felsites illustrate it as unmistakably as do the English felsites. 

Dr. Irving, 4 in his description of the Beaver Bay group of the Kewee- 
naw series, repeatedly calls attention to the resemblance between the 
ancient felsites and quartz-porphyries and the modern rhyolites, though 
he does not express an opinion as to their equivalence. 

1 Op. cit. Also Ueber arclueische Ergussgesteine aus Smaland; Bull. Geol. Inst Upsala, No. 2, 
Vol I, 1893, pp. 1-127. 

; Les ancienncs rhyolites, elites eurites, tin Grand-Manil : Bull. Acad. roy. Belgique, 3d series Vol. X, 
1885, pp. 253-315. Les eurites quartzeuses (rhyolites anciennes) de Nivelles et des environs: Bull. 
Acad. roy. des sci. et des beaux-arts de Belgique, 57 aunee, 3d series, Vol. XIII, No. 5. 1887. 

;/ M. E. Wadsworth, Notes on the mineralogy and petrography of Boston and vicinity: Proc Boston 
Soc. Nat. Hist., Vol. XIX (May, 1877), p. 236. On the classification of rocks : Bull. Mass. Comp. Zool. 
Harvard Coll., Vol. V, No. 13, June, 1879, p. 277. 

4 0p. cit., pp. 312, 313, note 5, p. 436. 



Messrs. Hague and Iddings l make the statement " that the degree of 
crystallization developed in igneous rocks is mainly dependent upon 
the conditions of heat and pressure under which the mass has cooled, 
and is independent of geological time." 

On the question of devitrification the writer finds no more direct 
expression of opinion, but the fact of devitrification is recognized by 
Iddings, Williams, Cross, and Diller. In none of the felsites elsewhere 
described have the varied structures of the modern rhyolites been more 
perfectly and conspicuously preserved than in the aporhyolites of the 
Eolith Mountain. 


The chemical study ot the South Mountain rocks has been rendered 
easy by a number of analyses of these rocks made by the late Dr. Genth, 
under the auspices of the Second Geological Survey of Pennsylvania. 
Analyses of the sedimentary, acid, and basic igneous rocks of South 
Mountain are distributed through pages 252 to 282 of Eeport COC. 

The analyses of the acid volcanics have here been brought together 
and tabulated. 

Table of analyses. 











Si0 2 

79. 970 

79. 920 



2. 950 


75. 570 




75. 31 




P 2 O fl 

A1,0 3 

13. 860 


Fe 2 Oi 



Ti0 2 











K 2 

3. 010 



Na.O .. 

H 2 







95. 038 


100. 77 

99. 09 

100. 06 

100. 65 

I. Fissile green schist. Between Pitie Grove Furnace ami Laurel forge. 2 

II. " Orthofelsite." "One-fourth of a mile north of Lerew's store." 3 

III. "Orthofelsite." ''Cut on turnpike 5 miles northwest of Petersburg, Cumber- 
land County." 3 

IV. "Laminated felsite." "East of Bigham Copper Mine." 4 

V. "Laminated orthofelsite. 7 ' "One-fourth of a mile southeast of Caledonia Fur- 

VI. 'Finely laminated orthofelsite." "One-fourth of a mile west of Cole's saw- 
mill, on the Shippenburgroad." > 

Their general uniformity and their close agreement with the analyses 
of typical rhyolitic lavas is the most striking feature of these analy- 

1 On the development of crystallization in tlio igneous rocks of Washoe, Nov., with notes on the 
geology of the district: Bull. U. S. Geol. Survey No. 17, 1885, p. 40. 

2 Analysis made by A. S. McCreath for commercial purposes ; alkalies undetermined. 

3 Analyses made by Dr. Genth and Henry Trimble, Second Geol. Surv. Pa., Vol. CCC, pp. 263-269. 

4 Analysis made by C. Han ford Henderson, of Philadelphia, The copper deposits of the South 
Mountain; Trans. Am. Inst. Mm. Eng., Vol. XII, 1884, p. 90. 



BULL. 136. 

ses, and is it convincing proof of the igneous origin of the rocks which 
they represent. In the absence of samples of the rocks analyzed or of 
exact descriptions of their character, special points can be brought out 
only inferentially. 

The high percentage of the alkalies and the slight trace of lime 
plainly denote the character of the feldspathic constituents. This 
indication of their chemical character coincides with the optical and 
physical properties enumerated on page 40. The rock from which 
Analysis IV was made is reixu'ted to be from the same locality from 
which many of the aporhyolites were obtained, the optical character of 
whose feldspars were tested. 

Microscopic study of these rocks leads us to expect a percentage of 
titanium oxide. In two instances the analyses show it. It is not 
unlikely that in the other cases the titanium oxide was not determined. 
The absence of manganese oxide from these analyses is surprising, as 
the Monterey porphyries and aporhyolites show a high percentage of 
it. The lime and magnesia present are doubtless due to the presence 
of epidote in the rock. The silica, alumina, and iron percentages are 
exactly normal and call for no remark. 

Analysis I, of a fissile schist, illustrates the slight change of chemical 
constitution which accompanies dynamic action in the acid rocks. 

Among the analyses made by Dr. Genth for the Second Geological 
Survey of Pennsylvania are the following, which, because of their 
anomalous character, have not been tabulated with the others : 

I. "Slaty rock." " Nine miles southwest of Dillersburg." 

II. "Purplish slaty orthofelsite." "One and one-half miles southeast of Mount 

These rocks plainly do not represent the normal type of the South 
Mountain acid rock. In the absence of specimens or means of deter- 
mining the character of the rocks from which these analyses were made, 
it is impossible to explain altogether satisfactorily the abnormally low 
percentage of silica and the high percentages of alumina and iron. 

If these slates were once normal aporhyolites, the shearing which 
produced the subsequent slaty character must have been accompanied 
by an abundant development of sericite from the feldspar. If the silica 



,v- M V Y ,r4- v ''wf^; 


r ifefjp 




From crest of hill east of the Clermont House. One-half natural size. The details of the brecciated structure 
are greatly obscured in the photograph. Some of the fragments are spherulitic ; many of them fit together, 
forming what v. as once a large fragment. The colors are shades of pink, red, and purple. 


thus set free was carried off by percolating* water, a low silica percent- 
age and a correspondingly high alumina percentage would result, whde 
the alkalies would remain about the same. The increase in iron may be 
due to infiltration. 



The presence of acid pyroclastics in the Monterey district has already 
been mentioned. Although a conspicuous feature of a portion of South 
Mountain, notably of the Buchanan Valley north of the Chambersburg 
turnpike, where they cover about 2 square miles, they play an insig- 
nificant role among the rocks of the Monterey district. Their charac- 
ter, however, is unmistakable. They may be classified as tuffs, flow 
breccia, and true breccia or tuffaceous breccia. 

This is a dark-purple banded rock, the clastic character of which is 
hardly evident in the hand specimen. Microscopic examination dis- 
closes its tuffaceous nature. Minute angular fragments, not exceeding 
a millimeter in length, are thickly distributed through a crystalline 

The fragments are usually spherulitic, with the replacement of the 
spherulitic crystallization, as in the massive aporhyolites described on 
page 52, by a fine mosaic, so that in ordinary light the spherulites are 
traceable only from their outline. In the same way, under crossed nicols 
a uniform granular crystallization obscures the fragmental character 
of the rock — a character which, in ordinary light, is sharply brought 
out by an outlining pigment of red iron oxide. Fragments of quartz 
and feldspar are among the inclusions. The groundmass, which is of 
the same chemical and mineralogical constitution as the included frag- 
ments, doubtless represents an ash recrystallized. 


These occur at widely separated localities — northwest of Old Maria 
furnace, near the source of Toms Creek, on the Gladhills road, and on 
the brow of the hill east of the Clermont House. These breccias are 
composed of fragments of considerable size, which plainly were caught 
in a viscous acid magma, as is evidenced by their linear arrangement 
in How lines and by the way in which different fragments fit together, 
forming what was once a larger fragment. On the weathered surface 
of the rock its brecciated character is rendered very manifest in the 
varying tints of pink, red, purple, and blue (PI. XII). The fragments 
range in size from the submacroscopic to those that are 2J inches in 
diameter. Their spherulitic character is discernible by the naked eye. 
Under the microscope, in ordinary light, the fragments frequently show 
either perlitic parting, a spherulitic character, or a regular arrangement 


of the coloring matter parallel to tlie boundaries of the fragments, due 
to water deposition. Grossed nicols again show uniform crystallization 
or a micropoikilitic structure. In one specimen of breccia this was not 
the case, however. A coarsely crystalline siliceous cement is quite 
distinct in grain from the uniformly finely crystalline fragments. This 
maybe a lava flow crushed and recemented. 


In some instances the groundmass doubtless represents an altered 
ash, when the rock becomes a true breccia. A breccia of this sort from 
the Monterey district has epidote largely developed in the matrix. 
The granulated quartzes and the perthitic feldspars of the included 
fragments show in a marked way the effect of dynamic action. 

At Raccoon Greek a fine specimen of breccia was found (PI. XIII), 
and in the Buchanan Valley breccia is extensively exposed. Some of the 
fragments contain chain spherulites. At Goles Gorner (Buchanan Val- 
ley), some G miles northeast of Graeffenburg, the breccia is sheared, 
and with the development of sericite the rock has become more or less 
slaty, Avhile still conspicuously retaining its brecciated character. 

The presence of these tuffs, flow breccias, and breccias proper, which 
are the natural accompaniments of surface lava flows, is inexplicable 
under any other hypothesis of the origin of the acid rocks. 


The alteration of the feldspathic constituent of quartz-porphyries to 
sericite or some other micaceous mineral, under the action of dynamic 
forces, has been frequently described. 1 

The production, in this way, from massive acid eruptives, of schists 
and slates resembling true porphyroids, 2 is finely illustrated in the 
South Mountain. In a single exposure (west end of Long Mountain, 
west of Gettysburg) felsiteswith distinct phenocrysts grade insensibly 
into a crinkled sericite slate. The shear zone is of limited width (10 
feet), and bounded on each side by massive felsites. The phenocrysts 
are only slowly obliterated and can be distinguished until the last stage 
in the alteration has been reached. 

Thin sections of five successive stages were studied. They show a 
development of sericite first around the feldspar phenocrysts and in a 
plane of dislocation. It is only sparingly developed in the ground- 

J J. Lehmann, Untersuchungen iiber die Entstehung der altkrystallinischen Schiefergesteine, Bonn, 
1884, Cap. IX, Druckschieferung und Glimmerbildung, p. 136. 

A. von Groddeek, Zur Kenntniss einiger Sericitgesteine, welche neben und in Erzlangerstatten 
auftreten : Neues Jakrbuch fur Mineral., etc. Supp. Vol. IV, 1886, p. 428. 

G. H. Williams, Bull. II. S. Geol. Survey No. 62, pp. 61, 121, 212. 

Bonney, On some nodular felsites in the Bala group of North Wales: Quart. Jour. Geol. Soc. Lon- 
don, Vol. XXXVIII, p. 289. 

Callaway, On the genesis of the crystalline schists of the Malvern Hills: Quart. Jour. Geol. Soc. 
London, Vol. XLI1I, pp. 530, 531. 

P. L. Milch, Beitrage zur Xenntnis des Verrucano, 1892, pp, 128, 129. 

2 Eosenbusch, Pet. Massigen Gesteine. Vol. II, 2d ed., p. 411. 




Natural size. The photograph does not bring out clearly the brecciated character of the work, which is 
very pronounced in the specimen. The matrix is apparently an ash containing angular fragments of a larger 
size (average diameter 1 cm.). The colors are blue, gray, and buff. 



mass. In the next stage the groundmass shows a decided, tendency 
to a parallel arrangement, and sericite is more abundantly developed. 
Eventually the phenocrysts are obliterated and there is much sericite 
in the groundmass. It is not, however, developed to the exclusion of 
the feldspar, while the quartz remains unaltered. 

In general, the development of sericite stands in direct relation to the 
shearing, and increases up to an almost complete, if not quite com- 
plete, replacement of the feldspathic constituents of the groundmass. 
Silica remains as a constituent of the groundmass. 

Material obtained from an artesian well at a point 40 feet below the 
surface furnished a similar shear zone, which displayed an even more 
abrupt transition from a porphyry to a sericite-schist. A single micro- 
scopic section included both porphyry and schist in typical develop- 
ment. The former showed an early stage of the alteration which was 
complete in the latter, where only sericite and a very schistose siliceous 
microgranitic groundmass remained. In this case, and in the extreme 
stage of the transition previously described, it would be impossible, 
with the microscope alone, to decide whether the schists were of clastic 
or nonclastic origin. This is one of the instances where field evidence 
is quite essential to the authoritative determination of the origin of the 

At the Bechtel shaft there has been thrown out a mottled red and 
white schist which has been produced by the shearing of a massive 
felsite. Here the phenocrysts have been replaced by a quartz mosaic 
and some sericite, which is also largely developed in the groundmass. 
The red mottling is due to a more or less parallel arrangement of red 
iron oxide globulites. 

A light green sericite-schist found on the railroad near Blue Eidge 
Summit station, and closely resembling some schists in situ exposed 
on the Gettysburg Railroad below the Clermont House, shows under 
the microscope phenocrysts of feldspar containing inclusions of a 
former glassy magma, still well preserved and showing twinning stria- 
tums. These phenocrysts occur in a groundmass of quartz, a little 
feldspar presumably, much sericite, epidote,ilmenite, or magnetite, and 

The color of the schist is due largely to the epidote. 

At the exposure just now mentioned on the Gettysburg Railroad, 
east of the Clermont House, there occurs a handsome, light, silvery- 
green, crinkled sericite-schist. Several rods to the north of this 
exposure the railroad cuts through quartz-x)orphyry,'but the contact 
between the porphyry and the schist is not exposed. The schist is sim- 
ilar to those already described, whose gradual passage into a massive 
porphyry could be followed in the field, and shows traces of pheno- 
crysts under the microscope, and in the hand specimen on the surface 
at right angles to the cleavage. The cleavage surfaces often display 
exquisitely delicate and manifold dendritic tracery. (PI. XIV.) 
Bull. 136 5 


On the highroad from Fountaindale to Fairfield, not far from the 
"Old Copper Shaft," occurs a dark purple-gray spotted slate. The 
light-green spots are sometimes irregular, but more frequently possess 
crystalline outlines and prove under the microscope to be a sericitic 
alteration of feldspar phenocrysts. Much of the feldspathic material 
still remains. The groundmass consists largely of iron oxide, which by 
its prevalence obscures the other constituents — leucoxene and quartz. 

Microscopic evidence is sufficient in this instance to determine the 
origin of the rock. It is plainly a sheared eruptive, and probably a por- 
phyritic aporhyolite, although the irregular outline of some of tbe 
sericitic areas suggests a brecciated aporhyolite. 

The occurrence of these slates is an interesting feature of the geology 
of the South Mountain. In the hand specimen they might readily be 
confused with a porphyroid, that is, a metamorphosed clastic rock, and 
have been so confused by geologists. They did not escape the atten- 
tion of Professor Rogers, who alludes to them as "the fissile talcose 
rock" near the "reddish gray rock, containing specks of reddish feld- 
spar," and includes them among the primal slates whose highly altered 
condition he repeatedly contrasts with the other slightly altered sedi- 
ments (sandstone). If these slates were of clastic origin, a high degree 
of metamorphism was necessary to produce their present crystalline 
condition, and Professor Bogers was quite right in drawing a contrast 
between their extreme metamorphism and the comparatively unaltered 
condition of all the other sediments. The very fact that their develop- 
ment from a sediment calls for such a high degree of metamorphism con- 
fined to limited and isolated zones, and for which no adequate cause can 
be assigned, renders such an origin as iuq^robable as it is unnecessary. 
Such a "selective metamorphism" is not demanded by the facts. 

As a matter of fact, these slates are scarcely more altered than the 
sandstone. Dynamic action in the latter has developed a quartzite. 
Dynamic action in the less resistant porphyry and aporhyolite has 
produced a sericite slate. That the chemical character of the acid 
rock remains essentially unaltered is evinced by analysis I, given on 
page 61. This shows exactly the composition of a rhyolite, and is 
totally unlike that of the sediments of the region. The sedimentary 
argillaceous slates of South Mountain are very little altered, and 
exhibit no tendency toward the development of porphyroids. 

All evidences — field relationship, successive stages shown in the 
hand specimen and under the microscope, chemical character, inherent 
improbability of clastic origin — combine to reveal the igneous character 
of these acid slates. 


The acid igneous rocks of the South Mountain have proved to be 
quartz-porphyries, devitrified rhyolites (aporhyolites) with accompany- 
ing pyroclastics, and sericite- schists. 




7» 4*. ■' 


4: ** 

'%* ■: 


## ?£ 


From exposure on the Gettysburg Railroad just below (east of ) the Clermont House. One-half natural size. It 

shows the dendritic markings which are frequently to be seen upon the cleavage surfaces of the acid slate. 

bascom.] SUMMARY. 67 

The first are typical noncrystalline porphyries, characterized by a 
soda-feldspar and by the presence in many cases of accessory piedmont- 
ite. The second group are the prototypes of the modern rhyolites, 
differing from them only in the loss of a vitreous base through devitri- 
fication. They are without phenocrysts, with inconspicuous pheno- 
crysts, and with abundant and conspicuous phenocrysts. Like the 
porphyries, they are characterized by a soda-feldspar — that is, they are 
of the pantellerite type. The evidence for devitrification lies in the 
abundant presence of structures peculiar to glassy lavas, in the present 
holocrystalline character of the rocks, and in the empirical knowledge 
that g-lass may become crystalline through lapse of time. 

The sericite schists are a metamorphic product of the first two 
classes by means of dynamic action. 

The alteration which the original types have undergone subsequent 
to consolidation is, in the case of the aporhyolites, devitrification 
(statical metamorphism); in the case of the schists, sericitization 
(dynamical metamorphism); and in the case of all three groups, includ- 
ing the quartz-porphyries, an epidotization (weathering). 




Many of the petrographical data upon which uniformitarian argu- 
ments have been based have been drawn from the comparative study 
of basic eruptives, and there is a marked disposition to disregard age 
in the nomenclature of these rocks. Among German petrographers, 
Keyer, 1 Tietze, 2 Eeiser, 3 Eeusch (H. H.), 4 and Sness r ° have supported the 
view that age is not a just ground of distinction between eruptive 
rocks, and Rosenbusch 6 predicts that in no very distant future the 
separation of effusive rocks into an older and a younger series "will 
prove untenable." English and American petrographers are practi- 
cally disregarding age m their nomenclature of the basic igneous 
rocks. Among the former, Judd, T Teall, 3 Allport, 9 Bonney, 10 Phillips," 
and Hobson 12 are notable. Among American petrographers, the 
Danas 13 and Iddings 14 have disregarded age in their usage of basic 
rock names. 

In plagioclase-augite rocks, the distinction between the gabbro and 
the diabase groups has been finally recognized as structural and 
not inineralogical, and the distinction between the diabase and the 

>E. Reyer, Beitriige zurFisikderErup., 1877, pp. 142-171 ; ref. Hussak : Neues Jahrbuch fur Minei al., 
etc., 1892, Vol. II, p. 147. Beitrage zurFisik der Erup. und der Eruptivgesteine 1887, p. 135. 

2 E. Tietze, Das Altersprincip bei der Nomenclatur der Eruptivgesteiue: Verhandl. k. k. geol. 
Eeichsanstalt, Wien, 1888, p. 166; ref. E. Becke: Neues Jalirbueh fur Mineral.. Vol. II, 1884, p. 303. 

3 Karl A. Eeiser, Ueber die Eruptivgesteine des Algaii: Tschermaks mineral. Mittheil., Vol. X, 
1889, pp. 500-550. 

4 H. H. Reuscb, Ueber Vulkanismus, Berlin, 1883, 

c Suess, Das Antlitz der Erde, Vol. I, pp. 204-206, 1883. 

6 H. Eosenbuscb, Ueber die ckemischo Beziebungen der Eruptivgesteine: Tscherniaks mineral. 
Mittheil., Vol. XI. 1890, p. 146. 

' Judd, On the gabbros, dolerites, and basalts of Tertiary age in Scotland and Ireland : Quart. Jour. 
Geol. Soc. London, Vol. XLII, 1886, pp. 49-97. The secondary rocks of Scotland: Quart. Jour. Geol. 
Soc. London, Vol. XXX, 1874, pp. 220-303. 

»Teall, Address of the president Geol. Sec. (e) of the British Assn. Adv. Sci., 1893. 

9 Allport, On the basaltic rocks of the Midland coal fields: Geol. Mag., Vol. VII, No. 70, 1870, pp. 
159-162 Tertiary and Palaeozoic trap rocks: Geol. Mag. Vol. X, 1873, p. 196. 

)0 Bonney, Quart. Jour. Geol. Soc. London, Vol. XXX, p. 529. 

"Phillips, On the so-called greenstones of central and eastern Cornwall: Quart. Jour. Geol. Soc, 
London, Vol. XXXIV, p. 471. 

12 Hobson, On the basalts and andesites of Devonshire : Quart. Jour. Geol. Soc, London, Vol. XL VIII, 
1892, pp. 496-507. 

13 J. D. Dana, On some points in lithology: Am. Jour. Sci., 3d series, Vol. XXXVIII, 1878, pp. 
336, 438. E. S. Dana, Trap rocks of the Connecticut Valley: Proc Am. Assn. Adv. Sci., 1884. 

14 Iddings, The columnar structure in the igneous rock on Orange Mountain, New Jersey: A.m. 
Jour. Sci., 3d series, Vol. XXXI, Mav, 1886, p. 331. 

bascom.] BASIC ERUPTIVES. 69 

basalt (dolerite) as one of the degree and granosity of the crystal- 
lization. It is easy to see that these features are determined by the 
geological conditions of consolidation. It follows from this that the 
essential characteristics of the rock groups are independent of age. 

A history of the classification of the gabbro and its allied groups 
and an exact statement of the final definition of these groups have been 
concisely given by Dr. Bayley. 1 He suggests that the group of mela- 
phyres and augite porphyrites will eventually be dispensed with, when 
the olivine diabases and the diabase "will take the position thus left 
vacant, and the plagioclase augite rocks will be found to occupy these 
places with respect to each other; the gabbros, the position of a deep- 
seated rock; the diabases, that of the corresponding noncrystalline 
effusive; and the basalt, that of the hypocrystalline equivalent." 
With this understanding of the use of the terms diabase and basalt, 
the South Mountain basic rocks fall into the diabase group. 

They are noncrystalline, effusive, plagioclase augite rocks, with or 
without olivine. They' thus possess the characteristics of the augite- 
porphyrites and melaphyres (diabase group). Tiiey are so fine-grained 
as to appear homogeneous in the hand specimen, yet show no evidence 
in the thin section of an originally hypocrystalline character. There is 
no proof for or against devitrification. In the absence of such proof 
their present noncrystalline character will be recognized in their nomen- 
clature as primary. 



The quartz-porphyries and aporhyolites in the Monterey district are 
limited to numerous small detached areas. The melaphyres and augite- 
porphyrites, on the other hand, occupy a large, irregular area, covering 
the valleys, the foothills, and the mountain flanks. Besides this area, 
which constitutes about one-half of the entire district, there are two 
small areas north of the old Maria Furnace, which are surrounded by 
the acid rocks, thus reversing the usual relation of the basic and acid 
eruptives. Along the State line and to the south of that line the dia- 
bases are massive or schistose, and inconspicuously amygdaloidal. In 
the Monterey district the amygdaloidal character of the diabases is their 
most marked feature. In the exposures on the Gettysburg Railroad 
narrow zones of inconspicuously amygdaloidal or nonamygdaloidal mel- 
aphyres grade above and below into consx)icuously scoriaceous rocks. 

Basic igneous slates occur at the west end of the Gettysburg tunnel, 
where they grade into massive diabases. They also occur on Colonel 
Benchoff's place, at a locality just north of Gum Spring, on a line 
northeast of the Blue Ilidge Summit station, and form a knoll south of 
the Fountaindale post-office. (See map, PI. I.) 

'The basic massive rocks of the Lake Superior region: Jour, of Geology, July- August, 1893, Vol. 
I, No. 5, pp. 433-456. 


There are a few other localities where slates occur. Where the slates 
were not studied in thin section their igneous origin has not been 
considered as proved. 

Ash beds, diabases crashed and recemented with epidote and quartz, 
are exposed along the Gettysburg Railroad in the cats west of the 
tun neh 

A tuffaceous breccia, composed of fragments so rounded as to appear 
water worn, was found at the head of Mime Branch. At the Russel cop- 
per mine and a few other localities epidosites are abundant. At the 
former place they are evidently vein material, and carry the native 
copper. In other localities they undoubtedly represent the last stages 
of decomposition and alteration of the massive or more often of the 
tuffaceous diabases. 


The augite-porphyrites and melaphyres vary in color from a slate- 
blue or purple to all shades and tones of green. Where epidote is the 
predominating alteration mineral the prevailing color is light yellowish- 
green 5 with chlorite or actinolite as the alteration products the color is 
a dark green. 

The most persistent and striking feature of the augite-porphyrites 
and melaphyres is their amygdaloidal character, to which allusion has 
already been made. 

Bowlders on the roadside and in the fields show a curiously rough 
and pitted surface, due to epidote or quartz amygdules brought out in 
relief by weathering. Sometimes the bowlders closely resemble con- 
glomerates composed of green or white oval pebbles; or the quartz 
amygdules, when perfectly spherical, mimic the spherulites of the 
acid rocks. 

The diabases (augite-porphyrites and melaphyres) are rarely mas- 
sive, usually schistose, sometimes slaty, and almost universally amyg- 

As the amygdules quickly respond to pressure, they furnish a deli- 
cate test of the degree of schistosity present in the rock. Macroscopic- 
ally the schistosity is otherwise more or less obliterated by subsequent 
epidotization or chloritization. 

Genuinely massive diabases are exceptional in occurrence and lim- 
ited in extent. Occasionally a mass of rock has moved as a whole, 
under pressure, and thus close to schistose or even slaty diabase the 
rock may retain its massive character. In these cases, since there has 
been no shearing, the vesicles are often perfect spheres, showing no 
elongation from inagmatic movement. This is notably the case just 
north of Gum Spring, on the Old Furnace Road. Quartz amygdules 
show conspicuously as round white spots on the fresh surface of the 
rock, which is a dark blue-gray, or are brought out in relief on the 
weathered surface, giving the rock the appearance of being riddled 


with shot. In other localities there is a greater diversity both in the 
shape and in the composition of the amygdules, 

There is frequently a zonal arrangement of three minerals — epidote, 
chlorite, and quartz — the light-green epidote being developed on the 
edge of the vesicle and surrounding the chlorite and quartz, which are 
successively developed in the interior Where the vesicles are large, 
epidote sometimes occurs in beautiful radiating crystals, not com- 
pletely filling the vesicle, and occasionally associated with crystallized 

It is doubtless to these amygdules that Professor Rogers refers when 
he describes the " decidedly crystalline" primal slates as containing 
"segregated specks and even half-formed geodes of epidote and other 
minerals," or "as gray slate spotted with epidote." Wherever there 
has been sufficient shearing to form a slate a micaceous mineral is formed 
in the vesicles. This micaceous mineral is either sericite, when the 
slate is conspicuously ornamented with greenish-white oval spots, or it 
is chlorite, when the slate is marked with brilliant dark-green oval 

The nonamygdaloidal diabase is always more or less schistose and 
frequently slaty. At the second railroad cut beyond Gladhill's switch 
it has a banded appearance, due to an alternation in color, purplish green, 
dark and light green rapidly succeeding one another. The rock is fine- 
grained. The form of the banding and the structure of the rocks as 
disclosed by the microscope suggest that the bands represent ash beds. 

At the west end of the tunnel there is a curious differentiation of 
the diabase in color and sensitivity to pressure. This differentiation is 
limited to an irregular band which suggests in many ways an intrusive 
dike. The apparent dike traverses the nonamygdaloidal diabase in a 
direction oblique to the schistosity of the latter, At one end it diverges 
and sends out a branch which inirsues a course vertically downward 
and disappears beneath the surface. 

Fragments of the schistose diabase are included within the dike. 
Its upper surface is somewhat amygdaloid al, the interior compact, and 
the lower surface bordered by a band of light-yellow epidote. This 
band is more irregular in outline than the upper surface. 

The dike is intersected by two systems of line parallel, or approxi- 
mately parallel, quartz veins. Parallel to one of these systems is an 
easy cleavage. 

The diabase just above and below the dike is finely schistose. This 
schistosity is parallel to the course of the dike, and is particularly 
remarkable above the dike, where it follows every curve of the latter. 
The color of the dike is purplish, and contrasts with the surrounding 
dark-green diabase. 

The obliquity of this seeming dike to the general schistosity of the 
diabase, its inclusions of fragments of the surrounding schist, its 
divergent branches, and the foliation of the diabase parallel to the 


band are all more readily explained on the supposition that we are deal- 
ing with a genuine igneous dike than in any other way. The structure 
of the rock under the microscope and its analysis (see Analysis III, 
p. 78) show that there is no essential difference in these characters 
between the band and the schistose diabase which it traverses. Its 
chemical composition differs only in the high percentage of iron which 
it carries. It is possible that for some cause there has been a local con- 
centration of iron (to which the color is due) within the limits of this 
band, which renders it harder than the surrounding diabase and enables 
it to resist pressure more successfully. Hence, while not yielding itself 
to the pressure which produced the schistosity of the diabase, it has 
also been the means of producing a foliation in the diabase parallel to 
itself. The only other tenable hypothesis is that it represents a later 
intrusive lava flow of the same general composition as that of the rock 
into which" it was intruded. The manner in which it grades into a 
finely vesicular rock on its upper surface, and the inclusions of frag- 
ments of a green diabase, would be explained by this hypothesis. Its 
resistance to pressure would be due to the same cause in either case. 

In only a few instances do the diabases show a porphyritical struc- 
ture apparent to the naked eye. The diabases have not infreqently 
suffered crushing, and are recemented with quartz, epidote, and hema- 
tite, the former minerals predominating. Veins of asbestos with quartz 
occur in the more epidotic diabase. 


Original structures. — There is a marked uniformity of structure and 
of mineral constituents in the South Mountain diabases. 

Unlike the aporhyolites, the porphyrites and melaphyres do not show 
the effects of magmatic movement. Their structure is universally the 
ophitic, which is produced only in a magma in a state of equilibrium. 
The vesicles also, as has already been noted, do not betray any fluidal 
movement. (PI. XXVIII, a.) 

Crystallization is fine-grained (see p. 69), corresponding to what has 
been called the u microophitic." That originally this microophitic struc- 
ture was associated with and passed insensibly into the hyalopilitic is 
not impossible, although subsequent processes of alteration, chief among 
which is silicification, have destroyed all trace of an unindividualized 

Shearing has obscured and sometimes obliterated the delicate ophitic 
structure through processes detailed later. Where dynamic action 
found relief in the crushing of the rock rather than in the production 
of a schist, the ophitic structure remains perfectly preserved in the rock 
fragments. The porphyritic structure is inconspicuous. Among the 
nonolivinitic porphyrites intratelluric crystallization is nearly absent. 
Feldspar and augite phenocrysts are rare. This characteristic, together 
with the widespread development of the amygdaloidal structure, allies 
these rocks to Bosenburch's spilite type. 


The olivinitic porphyrites, or true melaphyres, contain olivine as a 
constituent of the grouudmass as well as in the very plentiful porphy- 
ritical crystals. 

The distribution of the melaphyres is quite similar to that of the 
spilites. The history of the two types since consolidation has been the 
same, and they will be discussed together. 

The vesicular structure is a conspicuous feature of the melaphyres 
and spilites. They range from rocks almost as vesicular as a sponge to 
a compact rock containing* only a few scattered vesicles. These vesicles 
are filled with material furnished by percolating waters, and a solid 
amygdaloid is formed. 

The mineral nature of the amygdules will be described under the 
secondary constituents. The vesicles vary in size from microscopic 
dimensions to 5 centimeters in length and 3 in breadth. They are very 
significant, both of the amount of shearing and of alteration present 
in the rocks which they characterize, and they have undoubtedly been 
a factor in determining the character of both processes (pp. 74-75). 

Secondary structures. — The micropoikilitic structure, as has been noted 
on page 49, is occasionally present. It is found in those melaphyres 
and spilites which have been thoroughly silicified by infiltration. The 
secondary nature of the structure is very plain. The original structure 
(the ophitic) is so well preserved, in spite of the replacement of the 
mineral constituents, that in ordinary light the altered character of 
the rock is scarcely apparent (PI. XIX, a and b). Polarized light at 
once betrays the extent of the alteration. 

Where the schistose character of tlie rock is pronounced in the hand 
specimen, it is also a marked feature of the thin section. The con- 
stituents, which in these cases are for the most part secondary, are 
arranged with their longest axes at right angles to the pressure. 

There has been so complete a recrystallization of the rock as to 
obscure its original character. It has been repeatedly pointed out that 
under conditions of pressure igneous rocks acquire a degree of schis- 
tosity which renders it almost impossible to determine their true char- 
acter and to distinguish authoritatively between foliated traps and 
metamorphosed slates (elastics). In the schists under discussion their 
relation to undoubted porphyrites leaves no room for doubt as to their 
origin, nor is their alteration so extended as has been described in 
other localities. The structure of the South Mountain porphyrites is 
usually far less altered than is the case with the greenstone schists of 
the Menominee and Marquette regions. 

Original constituents. — It is to be expected that these aucient rocks, 
comparatively soft and extremely vesicular, exposed as they have been 
to pressure, percolating waters, and weathering, sbould exhibit altera- 
tion. It is surprising that the alteration has not been so complete as 
to obscure altogether the original structures and constituents. The 
original constituents of the rock— plagioclase feldspar, augite, olivine, 
titaniferous magnetite — are either present in a comparatively fresh con- 


dition or are represented by characteristic alteration products, which are 
often paramorplis of the original mineral. The former is only rarely 
the case, while the latter is the rule. 

The feldspar occurs both as porphyritic crystals and as a constituent 
of the groundmass. The crystals of the first generation are from 0.6 to 
0.8 millimeters in length to 0.2 millimeters in breadth. Those of the 
second generation are lath-shaped, 0.4 millimeters in length to 0.4 
millimeters in breadth, and condition the microophitic structure. They 
are both striated, and show undulatory extinction when any of the 
original substance remains. Usually the feldspars are altered to epi- 
dote and quartz, or they have been completely replaced by quartz, while 
their crystal outline is preserved by the iron constituent. 

It is doubtful whether augite is present otherwise than as a constitu- 
ent of the groundmass where it is allotriomorphic. It is universally 
replaced by the more stable amphibole minerals or by epidote or 
chlorite, and some porphyritic crystals of the latter minerals may 
represent augite phenocrysts. The chief alteration product of augite 
in the schistose porphyrites is actinolite. This mineral is not limited 
in its development to the augite outlines, and it thus obscures the 
ophitic structure. Olivine crystals of two generations are readily 
recognized by means of their characteristic form, their irregular frac- 
turing, and sometimes by their alteration products. Usually the olivine 
is altered to epidote. In a few cases (PI. XXVIII, b) the crystals are 
still sufficiently unaltered to respond to the optical tests for olivine. 

There is a large amount of an opaque black oxide in the porphyrites. 
Where this was tested the pow'der was found to be magnetic. This 
fact, the crystal form of some of the oxide, and the analyses of these 
rocks point to the conclusion that much of it is magnetite, though 
undoubtedly very titaniferous. Occasional rhombohedral forms and 
cleavages indicate that ilmenite is also present. Both these minerals 
have given rise to an abundant development of leucoxene (PI. 
XX VII I, b). 

In order of abundance the original constituents rank as follows: 
feldspar, augite, magnetite and ilmenite, olivine. 

Secondary constituents. — The processes of alteration have been greatly 
assisted by the open-textured, vesicular nature of the rocks, and the 
mineral character of the amygdules is indicative of the character of 
the alteration of the rock mass. The vesicles are almost universally 
tilled with one or two or all of three minerals: epidote, quartz, and 
chlorite. Whatever is the amygdaloidal filling is also the prevailing 
alteration mineral. If quartz fills the amygdules the rock mass is 
more or less completely silicified. The ophitic structure, while pre- 
served in outline, is replaced by the nncropoikilitic. Quartz, titaniferous 
magnetite, leucoxene, and some epidote constitute the rock, which is 
distinguished in the hand specimen by its blue-gray color and white 

One of the most common amygdaloidal fillings is epidote with a 


little quartz. In this case epidote is the predominating alteration 
product These rocks are recognized in the hand specimen by a light- 
green color and green amygdules. Feldspar, augite, and olivine have 
all been replaced by epidote. The material for this mineral has 
undoubtedly been furnished by the interaction of feldspar and augite, 
and also has been brought to the rock by percolating water from over- 
lying rocks. In the extreme phase of this alteration these rocks are 

In the presence of a larger amount of iron, actinolite is abundantly 
developed. There is also much free iron oxide, and the rock becomes 
dark green in color. Where there has been shearing movement, as in 
the case of the acid rocks, a micaceous mineral is developed. In the 
aporhyolites the mineral is sericite; in the porphyrites it is chlorite. 
In the incipient stages of the schistose structure chlorite occupies the 
center of the amygdules, with quartz and epidote filling the rest of the 
space. When the rock, is so schistose as to be fairly called a slate the 
amygdules are represented by brilliant dark-green spots and consist of 
chlorite only. 

Chlorite in turn becomes the prevaling alteration product. Often 
none of the original constituents remain. Actinolite, chlorite, epidote, 
and secondary silica are the invariable constituents of the "spotted 
greenstone schists." The actinolite and chlorite both blur the outlines 
of the original constituents and obliterate the original structure. These 
rocks are a medium green in color. 

The important secondary constituents of the porphyrites are quartz, 
epidote, actinolite, chlorite, and leucoxeue. The prevalence of one or 
the other of these alteration products can be determined in the hand 
specimen by means of the color of the rock and the character of the 
amygdules. With reference to the character of the alteration which 
they have undergone, the melaphyres and spilites thus fall into the fol- 
lowing groups: 

1. A blue-gray rock with quartz amygdules which do not show 
shearing. Under the microscope it shows an ophitic structure well 
preserved, and a silicified groundniass. Localities: Near Gum Spring, 
on the Old Furnace road, on Minie Branch, and along the Gettysburg 
Railroad . 

2. A light yellowish-green rock with epidote-quartz amygdules, and 
epidote as a prevailing constituent. The original structure is obscured. 
Localities: South of the Clermont House on the Gettysburg Railroad 
and at the Russel copper mine. 

3. A medium-green spotted schist. Chlorite is the prevailing min- 
eral. The original structure is more or less completely obscured. 
Localities: Along the State line at the west end of the tunnel and at 
many other places. This is a prevailing type of the porphyrite. 

4. A dark-green rock, more or less schistose. Epidote, quartz, and 
chlorite form the amygdules. Actinolite is abundant as an alteration 


product. Feldspar is often fresh and unaltered and the structure 

In the first three types actinolite may also be present, but not so 
abundantly as in the last. The first type passes into the second by 
increase in epidote, and the second type grades readily into the third 
by increase in chlorite. As this increase accompanies the development 
of schistosity, most of the schists belong- in the third group. 

Types 3 and 4 are not sharply separated. Chlorite and actinolite are 
present in both. In the former chlorite predominates, and in the latter 

Olivine may be present in any of the four groups. The crystal out- 
lines are best preserved in the first group; hence the ophitic structure 
is here best preserved; and it is most obscured in group 3, where orig- 
inal crystal outlines are lost. 

Group 1 contains the rocks which have been the least sheared, a fact 
which is perhaps accounted for by the silicified character of the rock. 

The peculiar dike-like band which traverses the basic eruptives at 
the west end of the tunnel is not unlike type 1 in color and compactness 
of texture. The color is several shades darker, and the specific gravity 
of the rock is greater. Under the microscope a further remsemblance 
is seen. The compact rock mass consists largely of titaniferous mag- 
netite (arranged in layers, or outlining obscurely an ophitic structure), 
chlorite, epidote, and quartz. 

The amygdaloidal selvage of the band shows the same constituents 
in an inverse proportion, and the ophitic structure is strongly marked. 
This was true in all the numerous thin sections made of the baud. In 
the vesicular portion of the band the ophitic structure is well pre- 
served, and even olivine crystals with unaltered outline are present. 
The amygdules are filled with quartz, granular and crystalline epidote, 
and chlorite. It is probable that the yielding of the vesicles, which 
would offer the least resistance to pressure, saved the rock. The 
tendency to a parallel arrangement of the feldspars on either side of 
the amygdules accords with this supposition. The passage of this 
baud, which is sometimes only from an inch to two inches wide, into a 
green chlorite-schist or slate is very abrupt. The difference seems to be 
due to the presence in the band of a large amount of iron. Its chem- 
ical analysis, given on page 78, coincides with this view. 

Epidote and quartz are by far the most abundant and widely dis- 
tributed of the secondary minerals. This is true not only of the basic 
eruptives, but also of the acid eruptives Avhen they are the prevalent 
alteration products and amygdaloidal filling, and where piedmontite, 
a member of the epidote group, occurs in macroscopic quantities. 
There seem to have been conditions favoring an extensive epidotization 
and silicification. Undoubtedly there has been within the rocks them- 
selves a mutual reaction between the decomposition products of feld- 
spar and augite resulting in the production of epidote. The fact that 


rarely some fresh feldspar still remains, while the augite is always 
decomposed, indicates that the decomposition products of augite have 
acted upon fresh feldspar, thus also developing epidote. But the feld- 
spars and augite of the porphyrites under discussion will not account 
for all the epidote and quartz present in them, composing, as they do, 
the amygdules, and in many cases the entire rock. The dip of the 
foliation planes of the basic ernptives indicates a thickness formerly 
much greater. A large amount of erosion of the igneous rocks has 
occurred since they were elevated to their present position. The water 
which percolated through this great thickness of igneous material 
brought with it the lime and alumina. 

It is plain that these processes of epidotization and silicitication took 
place not only while the porphyrites were being elevated, but have con- 
tinued since the cessation of all dynamic? action. The filling of vents 
and cracks by these materials, the fresh, unschistose character of the 
epidote and quartz in vesicles which themselves show the effect of 
squeezing, the presence of granular epidote in the schists and slates, 
all lead to this conclusion. While this is true, there are, on the other 
hand, amygdules of epidote where the fan-shaped radiating crystals 
of epidote have been broken and pulled apart in consonance with the 
alteration in the shape of the vesicle, and the spaces thus formed filled 
with silica. 

The nonvesicular character of the acid ernptives has saved them 
from so extended an epidotization as characterizes the basic ernptives. 
In the case of the amygdaloidal aporhyohtes of the Bigham copper 
mine, the same conditions which obtained with the porphyrites have 
operated to effect with them an extended development of epidote. 
While there is so complete an alteration in mineral constituents, there 
is surprisingly little change in structure. 

In this respect the South Mountain basic eruptives are a contrast to 
similar greenstones of the Menominee and Marquette regions. A com- 
parative study of the greenstones of the two regions shows that the 
Lake Superior rocks, while more altered in structure, possess feld- 
spars less altered than do the South Mountain greenstones. Calcite is 
much more abundant in the former rocks, and epidote in the latter, as a 
secondary product. 

Acessory minerals. — Copper occurs in microscopic quantities in the 
amygdules of the basic eruptives, just as it did in the amygdaloidal 
aporhyolites. This is true only of the amygdaloids from the various 
copper- mine localities described on pages 25-27. At these localities the 
carbonates of copper, malachite and azurite, occur as thin stains. The 
former sometimes forms crystals of considerable size in the vesicular 
cavities (one-fourth inch). A silvery-green asbestos occurs in some 
abundance in quartz veins penetrating the basic eruptives. It is 
plainly a secondary product. A finely divided red hematite is some- 
times quite conspicuous in the amygdules as a cementing material for 



[BULL. 136. 

the crushed porphyrites. It also occurs in crystalline form, and at a 
single locality (south of the Fountaindale turnpike, on the north Hank 
of Haycock Mountain) micaceous hematite occurs in veins. Calcite is 
rare. It is occasionally present in the amygdules or as vein material; 
and in the case of a single specimen, broken from a roadside bowlder, it 
almost completely replaces the substance of the rock. 

The range of minerals, original, secondary, and accessory, found in 
the South Mountain rocks is a very limited one. 


The analyses tabulated below, with the exception of No. IV, are col- 
lected from analyses scattered through the publications of the Second 
Geological Survey of Pennsylvania. For Analysis IV the writer is 
indebted to the courtesy of Professor Daniells, of Wisconsin University : 

Table of analyses. 






Si0 2 



14. 71 

48. 02 






37. 225 

37. 03 


C 24. 13 

) 19.03"" 


P 2 5 

A1 2 3 


( 44.82 

Fe 2 3 


Ti0 2 



K 2 




7 040 

15. 79 





100. 54 

99. 65 

100. 397 

98. 975 


1 Not determined. 

I. "Orthofelsite, containing epidote, llf miles west of Gettysburg." 1 

II. "Epidotic rock, 2-\ miles from Mount Alto furnace." l 

III. " Chloritic schist from Bechtel shaft." - 

IV. Differentiated band at the west end of the tunnel. 3 

V. " Variegated chlorite-schist with chlorite ( ?), one-half mile northeast of Pine 
Grove." l 

The characterization of these rocks by the Second Geological Survey 
is somewhat vague, and in the discussion of the analyses the writer 
is again hampered by the lack of baud specimens of the rock analyzed. 

The percentages are about those of the normal augite-porphyrites 
and melaphyres as given in Koth's tables. They scarcely show as 
much variation as the altered augite plagioclase rocks of his tables. 

Although the analyses show some phosphorous pentoxide, no apatite 
was noted in the microscopic study. The iron percentage is high in all 
of the analyses, though not abnormally so — not higher than the micro- 
scopic study would lead us to expect. 

'Second Geological Survey of Pennsylvania, Vol. CCC, pp. 255-275. Analyses made by the late Dr. 
F. A. Genth. 

2 Frazer: Hypothesis of the structure of tho copper belt of the South Mountain, p. 82 : Trans. Am, 
Inst. Mm. Eng..Vol. XII, pp. 85-90. Analysis made hy C. Hanford Henderson. 

s Analysis made by Prof, W. W. Daniells, of Wisconsin University, 

bascom.] BASIC SLATES. 79 

The "epidotic rock" (I aud II) shows, as would be expected, an 
abnormally high lime percentage. 

The chief variation from the normal type lies in the addition of lime 
aud iron oxide and the abstraction of the alkalies aud magnesia. 



The localities where the basic slates occur in any considerable extent 
are colored a light yellow-green on the map of the Monterey district. 
The slight rounded eminence opposite the Fountaindale post-office is 
composed of a dark-gray crinkled and finely laminated slate. It is 
darker colored than the clastic slate of the region and is surrounded 
by the basic eruptives. The thin section shows very distinct traces of an 
ophitic structure. Iron oxide, chlorite, and sericite are the only con- 
stituents that can be determined. About three-fourths of a mile north 
of Monterey station, on the road leading from the turnpike to the Old 
Furnace road, there is an exposure of a lighter-colored slate. The same 
constituents are found in this slate, with a larger proportion of sericite 
and the addition of leucoxene. The ophitic structure is barely discern- 
ible, and it is with considerable hesitancy that the slate is referred to 
the group of igneous rocks. Half a mile farther northeast, on the Old 
Furnace road, just beyond Gum Spring, occurs a light-gray spotted 
slate of undoubted igneous origin. It is not so finely foliated as the 
slates that have just been described, and much of the original structure 
remains. The constituents are the same as those of the last-mentioned 
slate. Leucoxene is more abundant and the ophitic structure is pro- 

The amygdules, which give the slates a spotted appearance, are com- 
posed of quartz, sericite, and some chlorite. These slates are related 
to the porphyrites of Group I, some typical examples of which occur 
near by. 

At the west end of the Gettysburg tunnel, just above the iron-bear- 
ing band previously described, another spotted slate has been devel- 
oped from the basic igneous rock. In this case the slate is green, and 
the spots are a brilliant dark shade of the same color. Iron oxide, 
chlorite, epidote, and some silica are the constituents. The original 
structure is entirely obliterated. Some of the epidote grains faintly 
suggest olivinitic forms. Chlorite is the prevailing mineral. The spots, 
which as in the other slate are sheared amygdules, are formed of chlo- 
rite only. This slaty zone is only a few inches wide and passes some- 
what abruptly into a slightly schistose porphyrite. 

The first tunnel on the old Tapeworm .Railroad, which was abandoned 
before an excavation was made, exposes a green slate which differs from 
the one just described only in its nonamygdaloidal character and in the 
greater abundance of epidote. The knoll northeast of Blue Eidge 


Summit station, the roadway crossing the Gettysburg- Railroad in front 
of the Clermont House, and the hill to the east of the Fairfield-Foun tain- 
dale road and north of the Fountaindale turnpike, are the other locali- 
ties where slates occur. While their association and their appearance in 
the hand specimen, which resembles that of the first two slates discussed, 
indicate an igneous origin, in the absence of thin sections that origin 
can not be considered beyond question. 



The basic breccias of South Mountain may be classified as follows : 
(1) Crushed porphyrites which have been recemented with epidote and 
quartz and sometimes brilliantly colored with red hematite; (2) Tuffa- 
ceous breccia j (3) Ash. 

(1 ) Crushed porphyrites. — The crushed and broken porphyrites, while 
perhaps not in a strict sense breccias, present a strikingly brecciated 
appearance. Fragments of all sizes, of a blue or purple-gray rock, are 
embedded in a bright-green and white or rose-colored matrix. The 
fragments have undergone either silicification or epidotization without 
affecting materially their structure (microophitic), which is outlined 
by iron oxide. 

(2) Tuffaceous breccia. — Some large bowlders found near the source 
of Minie Branch furnish the only unmistakable tuffaceous breccia. 
The fragments show a considerable range in size (see page 24) and are 
thickly crowded in a basic cement. As there has been no shearing, 
the structure of the fragments is perfectly preserved. (PI. XXVIII, a.) 
Epidote, quartz, and iron oxide are their present constituents. 

(3) Ash. — Above the Headlight copper mine on the Fountaindale 
turnpike and in the fourth cut beyond Monterey station (northeast), on 
the Gettysburg Railroad, are intercalated bands of a light-green rock 
w 7 hich, for the following reasons, have been considered altered ash: 
At the west end of this cut a fine-grained homogeneous rock is striped 
with alternating bands of light green and reddish green. Under the 
microscope these bands show no trace of any structure save a slight 
schistosity. They are composed almost wholly of angular grains of 
epidote, magnetite and leucoxene, actinolite, chlorite, and quartz. The 
difference in color is due to the presence in the reddish bands of red 
iron oxide. Toward the eastern end of the same cut the whole face of 
the rock is banded with light-green epidotic layers from 1 foot to 2 
feet wide, running approximately parallel to one another. 

A microscopic slide of one of these bands consists wholly of granular 
epidote and quartz, with a little iron oxide, usually the red oxide. 

These rocks overlie and are in close proximity to scoriaceous basic 
lava. This fact, together with their variation in color and their struc- 
tureless and fragmental character, is very suggestive of an altered ash. 
At the first locality mentioned, the Headlight copper mine, the so- 

bascom] SUMMARY. 81 

called ash occurs as a light-green schist. It is not banded, but pre- 
sents under the microscope the same structureless character as the 
rocks abovp described. It consists of actinolite blades and needles, 
epidote granules, and some chlorite, magnetite, and leucoxene. 


The basic igneous rocks display but little variety of structure or 
mineral constitution. The former is that common to porphyrites and 
melaphyres — the microophitic — and in spite of great alteration in the 
mineral constituents of the rocks it still remains a marked structure. 
Shearing obscures it, but in the extreme form of the sheared porphy- 
rite — the slate — it is still discernible. 

The formation of chlorite and actinolite tends to confuse outlines; 
hence some of the. slates in which there is not much of these minerals 
preserve their original structure better than the chlorite-schists. The 
original mineral constituents, i)lagioclase, feldspar, augite, and olivine, 
have almost completely disappeared. No augite remains; olivine crys- 
tals are well preserved in outline, and sometimes a core of the original 
mineral remains. There is considerable feldspar still unaltered. It is 
always striated, but the crystals are too small to allow of an accurate 
determination of their character. That they belong to the basic end 
of the series is shown by their extended alteration to epidote, and by 
the chemical analyses of the rocks. 

The vesicular character of these rocks has aided in the extended 
replacement of their original minerals, and the amygdules are an index 
of the character of that replacement. The presence of vesicles has also, 
doubtless, been a factor in preserving the internal structure of the rock 
in spite of dynamic action. Silicification, epidotization, and chloritiza- 
tion are the processes of alteration which have been most active. 

The source of material is twofold — from the rocks in which these pro- 
cesses have been described, and from the overlying rocks which have 
been removed by erosion. 
Bull. 13G 6 



The preceding chapters have treated in detail structural and chemical 
features possessed by the South Mountain rocks and characteristic of 
igneous rocks only. These are regarded as sufficient evidence of the 
igneous origin of the rocks which they characterize, without further 
proof on that point. Jt only remains to show on what grounds a sedi- 
mentary origin has been attributed to them, and to sum up the evidence 
against such an origin. 


Schistosity. — The conformity of the foliation planes of the porphyries, 
the aporhyolites, and the porphyrites, with the foliation planes of the 
Cambrian sediments is a prominent and j>ersistent feature, and one 
which, among others, has undoubtedly led to the ascription of a sedi- 
mentary origin to the former rocks. The confusion of foliation planes 
and bedding planes in the quartzite, owing to the obscurity of the 
latter, added force to this argument. 

This conformable schistosity is not, of course, inconsistent with the 
igneous origin of the underlying rocks. The schistosity is a secondary 
feature, produced by forces which affected the igneous and aqueous 
rocks alike. 

The cleavage is quite as plainly secondary in the clastic rocks as in 
the nonclastic, and sometimes conforms to the bedding and sometimes 
does not. 

Lamination. — An original lamination, conspicuous in the aporhyo- 
lites, the nature of which was described on pages 43-44, characterized 
as bedding by Hunt and others, doubtless furnished another reason for 
attributing stratification and an aqueous origin to the rocks possessing 
it. The real nature of the lamination has proved to be such that it 
becomes an evidence of the igneous character of the rocks in which it 

That the lamination is due to bands of spherulites has been pointed 
out. True spherulitic crystallization, such as has been described in 
these aporhyolites, has thus far been known only as the product of crys- 
tallization from a molten magma. 

The slates. — The slaty character of the rocks has been another reason 
for assigning a sedimentary origin to them. 

The slates, both acid and basic, have been considered clastic slates 
by Professors Rogers and Lesley, by Frazer, Tyson, Blandy, and Dr. 


Hunt, as the quotations from these writers, given in Chapter I, show. 
While the igneous slates of South Mountain do not resemble any of 
the Cambrian sediments of that region, their resemblance to porphy- 
roids from regions where there has been extended lnetamorphism is 
very great. This resemblance is internal as well as external, and fur- 
nishes an instance of the production of essentially similar results by 
either of two different methods. 

The true nature of these slates and the manner of their production 
are conclusively revealed through held evidence. Where a single expo 
sure shows a shear zone of not more than 20 feet in which every gra- 
dation from a porphyry to a fissile slate is displayed, or where a single 
hand sx)ecimen shows such a metamorphism, the evidence of such a 
genetic relationship is irrefutable. 

Absoire of gradation between igneous and clastic rocks. — There is, on 
the other hand, no such gradation between the igneous rocks and 
undoubted elastics as might be expected if the former were metamor- 
phosed elastics. We have holocrystalline rocks sharply separated 
from noncrystalline elastics, with an entire absence of intermediate 

Professor Kogers was impressed with the high degree of metamor- 
phism which these rocks must have undergone in order to attain their 
present holocrystalline character. "A gray siliceous altered rock," "a 
compact siliceous altered slate," are the terms he uses to describe the 
porphyries and aporhyolites, while he speaks of the porphyrites as 
"primal slate in a highly metaniorphic condition" and "highly altered 
greenish slate." The sediments and the igneous rocks have been sub- 
jected to the same dynamic forces, and, as a matter of fact, we find 
one no more highly metamorphosed than the other relatively to their 
respective powers of resisting alteration. 

Surface-flow features. — Positive field evidence for the non sedimen- 
tary origin of these rocks is found m the features which they possess in 
common with surface flows. Their vesicular, scoriaceous, and pumi- 
ceous character, the accompanying pyroclastics, their flow structures, 
even grain, conchoidal fracture, and other characteristics of a glassy 
lava all testify to an eruptive origin. 


Structural. — The petrographical evidence of the origin of the porphy- 
ries, aporhyolites, and porphyrites is of an even more unmistakable 

Their porphyritic structure is indicative of their origin. Olivine and 
feldspar phenocrysts with crystalline outlines, idiomorphic quartz with 
embayments and edges rounded by magmatic corrosion, are possible 
only in rocks which have once been molten. 

The ophitic structure, preserved in great perfection in the porphy- 
rites, is peculiar to rocks which have consolidated from a molten magma. 


Other structures which furnish additional and convincing proof of an 
igneous origin need only be mentioned. A detailed description of their 
appearance in the aporhyolites and porphyrites has been given in the 
previous chapters. Such structures are the spherulitic, axiolitic, litho- 
physal, perlitic, rhyolitic, tiuidal, and amygdaloidal. 

Miner alogical. — Metamorphosed sedimentary rocks are always accom- 
panied by certain characteristic minerals. The absence of such minerals 
in these South Mountain rocks is conspicuous. Epidote and sericite 
are the only prominent alteration minerals. Of these, the former is the 
product of weathering rather than a true metamorphic mineral; the 
latter is more or less limited to shear zones, where its development is 
directly related to the dynamic force acting upon massive porphyries 
and aporhyolites. The absence of all evidence of contact action indi- 
cates their effusive character. 

Chemical. — The close conformity of the composition of these rocks in 
the one case with that of the rhyolites and in the other with that of the 
diabases and melaphyres from all parts of the world, as tabulated by 
Roth, indicates an igneous origin. Their uniform composition is a con- 
trast to the composition of a series of clastic rocks, where the chemical 
proportions are largely a matter of accident. A similar test has been 
applied by Eosenbusch l to the determination of the origin of Archean 
gneisses. The association of these types of acid and basic lava accord 
with the laws of petrographical consanguinity. 



The acid-lava flows in South Mountain are regarded by the writer as 
quite comparable, at the time of their consolidation, to similar flows in 
post-Tertiary time, such, for instance, as those which have been recently 
studied in the Yellowstone National Park. Certain portions of the flow, 
as in the case of the Obsidian Cliff, were completely vitreous save for 
spherulitic and lithophysal crystallization. In other localities the lava 
was lithoidal, and in the central portion of thick flows holocrystalline. 
In this way three types of acid volcanics would be developed — 
rhyolites, lithoidal rhyolites, and quartz-porphyries. Every gradation 
between these types would accompany these. 

Thus, while there are certain areas in the South Mountain, notably 
the JBighain Copper Mine and Eaceoon Creek localities, which exhibit 
typical ancient rhyolites, other regions display genuine quartz-por- 
phyries. While in the latter rocks, which constitute a not inconsider- 
able portion of the acid flows, the groundmass may have been, and prob- 
ably was, originally noncrystalline, as in some modern lavas, m the case 
of the former rocks it is supposed that the groundmass was, at the time 
of consolidation, wholly or partly glassy. 

•Zur Auffassung der chemischen Natur dea Grundgebirges: Tscherniaks mineral. Mittheil., Vol. 
XII, 1891, pp. 49-61. 


Chief among the processes of alteration which have been going on 
since that time is devitrification. Out of glassy and lithoidal rhyolites 
devitrification has been developing aporhyolites. This process consists 
in the replacement of whatever glassy base was present in the original 
rock by a uniform quartz -feldspar mosaic. Sometimes the alteration is 
carried still further, and the original spherulitic crystallization is also 
replaced by this secondary granular crystallization. Rarely, if ever, 
does the secondary crystallization completely obscure the former char- 
acter of the rock, while often all of the structures peculiar to a fresh 
glassy lava are retained. 

The other processes of alteration, which all of the original rock 
types have undergone in some slight degree, and some of them in an 
extreme degree, are the processes of sericitization and epidotization. 
The former process, has been a chief factor in the development of slates 
from massive porphyrites and aporhyolites. 

These three processes of alteration — devitrification, sericitization, 
and epidotization — represent statical metamorphisin, dynamic meta- 
morphism, and weathering, respectively. 


In contrast with the acid rocks, the basic rocks have their original 
constituents so completely replaced that it is not easy to determine the 
original type or types. The original constituents were plagioclase, pyr- 
oxene, olivine, ilmenite, and magnetite. The original structures were 
the microophitic, the porphyritic (inconspicuous and mostly confined to 
the olivine-bearing type), and the amygdaloidal, a universal structure. 

In view of these constituents and structures, these rocks have been 
regarded as members of the diabase or augite-porphyrite group and of 
the olivine-diabase or melaphyre group. The augite-porphyrites resem- 
ble the spilites in their scanty porphyritic crystals, their ever-present 
inclination to the amygdaloidal structure, and their susceptibility to 

The character of the alteration which has taken place in these mela- 
phyres and spilites varies with the amount of shearing which has 
occurred. Where shearing has been a factor in the alteration, chlorite, 
actinolite, and quartz replace the pyroxene and plagioclase. 

In the absence of shearing, epidote has resulted from the interaction 
of plagioclase and pyroxene. Olivine has altered to epidote, serpen- 
tine, and iron oxide ; ilmenite has altered to leucoxene, and magnetite to 
hematite, when altered at all. In the absence of shearing the ophitic 
structure is preserved in outline, although sometimes the micropoiki- 
litic is added to it through the infiltration of silica; with the presence 
of shearing the development of chlorite and actinolite has obliterated 
the original structure and producer! the schistosity characteristic of 
chlonte-actinolite rocks. 



These South Mountain volcanics form a part of a belt of similar 
rocks which have been recently recognized along- the Eastern Coast of 
the United States and Canada. 

Such volcanics have been described in New Brunswick by the Cana- 
dian geologists — Bailey, 1 Matthew, 2 and Ells. 3 

More recently, in the Sudbury district, similar rocks have been ob- 
served by Bell. 4 They have been described in Maine by Shaleiy' and ate 
recognizable in Canada, Maine, and New Hampshire through the 
writings of Hunt, Jackson, and Hitchcock, although they were other- 
wise interpreted by these observers. 

Spherulitic volcanics have recently been definitely recognized by W. 
S. Bayley G at Vinal Haven, Maine, and have been studied in detail by 
Mr. G. O. Smith, of Johns Hopkins University. 

They have been identified in the Boston Basin by Wadsworth 7 and by 
Diller, 8 who has studied them in some detail. The thin sections loaned 
by the latter for comparative study have already been mentioned as 
showing a marked similarity to the South Mountain acid volcanics. 

The continuation of the South Mountain volcanics in Maryland and 
Virginia has been studied by Keith. 9 Similar volcanics have been found 
in North Carolina by Professor Williams, 10 in South Carolina by Lieber, 11 
and in Georgia by Professor Pirsson. 12 

In Canada, Maine, in the neighborhood of Boston, and in Missouri 1 ' 
the felsites were, like the South Mountain rocks, first regarded as sedi- 
mentary in origin, and have only recently been identified as volcanic. 

With continued petrographic investigation of thepre-Cainbrian rocks 
of North America volcanics may yet be recognized at other points where 
the rocks have been interpreted as sedimentary. 

In the Lake Superior region they have long been known through the 
writings of Irving and others, and their extent in that region has 
recently been still further enlarged. 14 

'Bailey, Report on the pre-Silurian rocks of South New Brunswick : Rept. Can. Geol. Survey, 1877-78 
D. D. 

^Bailey, Matthew, and Ells, Report on Southern New Brunswick : Rept. Can. Geol. Survey, 1878-79. 
3 Ells, Volcanic rocks of Northern New Brunswick: Rept. Can. Geol. Survey, 1879-80, D. 
<Ibid., 1889-90, F, 1891. 

'Shaler, Cobscook Bay, Maine: Am. Jour. Sci. (3), Vol. XXXII, 1886, p. 40. Mount Desert: Eighth 
Ann. Rept. U. S. Geol. Survey, 188G-87, pp. 104:3, 1054. 
«Bull. Geol. Soc. Am., vol. 0, 1894, pp. 474-470. 

'Wadsworth : Bull. Mus. Comp. Zool. Harvard Coll., Vol. A r , No. 13, p. 282. 
'Diller, op. cit. 
'Keith, op. cit. 

10 For full statement of distribution of volcanic rocks on Atlantic Coast, see paper by Professor Wil- 
liams in Jour, of Geology, Vol. II, No. 1, pp. 1-31. 
"Lieber, Report on the survey of North Carolina, 1850, 2d ed., 1858, p. 31. 
12 A section of a Georgian felsite, loaned by Professor Pirsson, has already been alluded to. 
13 Haworth: Am. Geologist, Vol. I, 1888, p. 280; Bull. Missouri Geol. Survey, No. 5. 
14 U. S. Grant, Volcanic rocks in the Keewatin of Minnesota: Science, Vol. XXIII, Jan. 12, 1884, 
p. 17. 

basoom.] LITERATURE. 87 

The reported rarity of volcanic action 1 in America in pre-Oambrian 
times is perhaps more apparent than real, and is due rather to the failure 
to recognize the results of such action than to the actual absence of vol- 
banic action. 


A list of papers in which points of resemblance between ancient and 
modern acid volcanics have been emphasized, or in which devitrifica- 
tion has been described, and a list of articles on the nature and origin 
of spherulites, are appended. 


Allport, S. On the microscopic study of the pitchstoues unci felsites of Arran. 
Geol. Mag., Vol. IX, 1872, pp. 536-545. 

. On ancient devitrilied pitclistones and perlites from the Lower Silurian dis- 
trict of Shropshire. Quart. Jour. Geol. Soc, Loudon, Vol. XXXIII, 1877, p. 449. 

Bayley, W. S. Spherulitic volcanics at North Haven, Maine. Lull. Geol. Soc, 
Am., Vol. VI, 1894, pp. 474-476. 

Blake, J. F. Ou the felsites and conglomerates between Btdhesda and Llanllyfni, 
North Wales. Quart. Jour. Geol. Soc, London, Vol. XLIX, 1893, pp. 441-465. 

Bonney, T. G. On certain rock structures as illustrated by pitchstoues and fel- 
sites in Arrau. Geol. Mag., Vol. IV, 1877, pp. 499-511. 

. Note on the felsite of Brittadon, North Devonshire. Geol. Mag., Vol. V, 1878, 

pp. 207-209. 

. On some nodular felsites in the Bala group of North Wales. Quart. Jour. 

Geol. Soc, London, Vol. XXXVIII, 1882, p. 289. 

Boriiemann, T. G Die Quartzporphyre von Heiligeustein und seine Fluidalstruc- 
tur. Zeitschr. Deutsch. geol. Gesell., Berlin,Vol. XXXIX, 1887, p. 793. 

Chapman, F. On a method of producing perlitic and pumaceous structures in 
Canada balsam. Geol. Mag., Vol. VII, 1880, p. 79. 

Clements, J. Morgan. The volcanics of the Michigamme District of Michigan. 
Jour. Geol., Vol. Ill, No. 7, Oct.-Nov., 1895, pp. 801-822. 

Cole, Grenville A. J. On the artificial production of the perlitic structure. 
Geol. Mag., Vol. VIII, 1880, p. 115. 

. On hollow spherulites and their occurrence in ancient British lavas. Quart. 

Jour. Geol. Soc, London, Vol. XLI, 1885, p. 162. 

. On the alteration of coarsely spherulitic rocks. Quart. Jour. Geol. Soc, 

Loudon, Vol. XLII, 1886, p. 186. 

. On the devitrification of cracked and brecciated obsidian. Min. Mag., 

Vol. IX, No. 44, 1891, pp. 272-274. 

Cole, Grenville A. J., and Butler, G. W. On lithophysa; in the obsidian of the 
Roccho Kosse, Lipari. Geol. Mag., Vol. IX, 1892, p. 488. 

. On the lithophysa' in the obsidian of the Kocche Rosse, Lipari. Quart. 

Jour. Geol. Soc, London, Vol. XLVIII, 1892, p. 438. 

Davies, Thos. Preliminary note oh old rhyolites from Bouley Bay, Jersey. Min. 
Mag., Vol. Ill, 1880, pp. 118-119. 

Diller, J. S. Felsites and their associated rocks north of Boston. Proc Boston 
Soc. Nat. Hist., Vol. XX, 1880, pp. 355-368; Bull. Mus. Comp. Zool. Harvard Coll., 
Vol. VII, 1881, pp. 165-178. 

Futterer, Karl. Der Ganggranit von Gros-Sachsen und der Quartzporphyr von 
Thai im Thiiringer Wald. Inaug. Disser., 1890. 

Dana, J. 1)., Manual of Geology, 4th ed., 1895, p. 938. 


Hague, Arnold, and Iddings, J. P. The development of crystallization in the 
igneous rocks of Washoe, Nev., with notes on the geology of the district. Bull. 
U. S. Geol. Survey, No. 17, 1885. 

Harker. Bala volcanic series of Caernarvonshire and associated rocks. Sedg- 
wick prize essay for 1888. Cambridge, 1889. 

Hatch, F. H. On the Lower Silurian felsites of the southeast of Ireland. Geol. 
Mag., Vol. VI., 1889, pp. 545-549. 

Irving, R. D. The copper-hearing rocks of Lake Superior. Mon. U. S. Geol. Sur- 
vey, Vol. V, 1883. 

Judd, J. W. On composite dikes in Arran. Quart. Jour. Geol. Soc. London, Vol. 
XLIX, 1893, pp. 536-565, pp. 546, 551, 560. 

Kalkowsky, Ernst. Mikroskopische Untersuchungen von Felsiten und Pech- 
steinen Sachsens. Tschermaks mineral. Mittheil., 1874, pp. 31, 58. 

Klockmann, F. Der Geologische Aufbau des sogen. Magdeburger Uferrandes 
init hesonderer Berlicksichtigung der auftretenden Eruptivgesteine. Jahrbuch K. 
preuss. geol. Landesanstalt, Vol, XI, 1890, pp. 171-203. 

La Croix, Alf. Comptes-rendus, CXI, p. 71. 

Lang, Heinrich Otto. Grundriss der Gesteinekunde, 1877, p. 43. 

Levy, A. Michel. Caracteres microscopiques des roches anciennes acides, con- 
siders dans leurs relations avec Page des eruptives. Bull. G6ol. Soc, France, Feb., 

Lossen, K. A. Beitriige zur Petrographie der plutonischen Gesteine. Abhandl. 
K. Akad. Wiss. zu Berlin, 1869, p. 85. 

McMahon, Lieut. Gen. C. A. Notes on some trachytes, metamorphosed tuffs and 
other rocks of igneous origin on the western flank of Dartmoor. Quart. Jour. Geol. 
Soc, London, Vol. L, 1894, p. 338. 

Milch, L. Beitriige zur Kenntnis des Verrucano, Leipzig, 1892; Part 2, 1896. 

Miigge, O. Untersuchungen uber die " Lenneporphyre" in Westfalen und den 
angrenzenden Gebieten. Neues Jahrb, fur Min. Geol. u. Pal., Vol. VIII, 1893, pp. 

Nordenskjold, Otto. Zur Kenntniss der sogen. Halleflinta des nordostlichen 
Smalands. Bull. Geol. Inst., Upsala, No. 1, Vol. I, 1893. 

. Ueber archreische Ergussgesteine aus Smaland. Bull. Geol. Instit. Upsala, 

No. 2, Vol. 1, 1893, pp. 1-127. 

Osann, A. Zeitschr. Deutsch. geol. Gesell., Berlin, Vol. XLIII, 1891, pp. 691, 716. 

Rosiwal, A. Petrographische Notizen iiber Eruptivgesteine aus dem Tejrovieer 
Cambrium. Verhandl. K. k. geol. Reichsanstalt, 1894, p. 210. 

Rutley, F. Perlitic and sperulitic structures in the lavas of Glyder Fawn. Quart # 
Jour. Geol. Soc, London, Vol. XXXV, 1879, p. 508. 

. On the microscopic structure of devitrified rocks from Beddgelert and Snow- 
don. Quart. Jour. Geol. Soc, Loudon, Vol. XXXVII, 1881, p. 403. 

. The microscopic character of the vitreous rocks of Montana. Quart. Jour. 

Geol. Soc, London, Vol. XXXVII, 1881, pp. 391-402. 

. On strain in connection with crystallization and perlitic structure. Quart. 

Jour. Geol. Soc, London, Vol. XL, 1884, pp. 340-346. 

. Felsitic lavas of England and Wales. Geol. Survey of England and Wales, 


. Notes on alteration induced by heat in certain vitreous rocks. Proc Royal 

Soc, London, No . 245, 1886. 

. On some eruptive rocks from the neighborhood of St. Minver, Cornwall. 

Quart. Jour. Geol. Soc, London, Aug., 1886, pp. 392-401. 

. On the rocks of the Malvern Hills. Quart. Jour. Geol. Soc, London, Vol. 

XLIII, 1887, p. 499. 

. On perlitic felsites and on the origin of some epidogites, Quart. Jour. Geol, 

Soc, London, Vol. XLIV, 1888, pp. 740-744. 

bascom.] LITERATURE. 89 

Rutley, F. On tachylite from Victoria Park, Whiteinch, near Glasgow. Quart. 
Jour. Geol. Soc, London, Vol. XLV, 1889, pp. 623-632. 

. On composite spkerulites in obsidian from Hot Springs, near Little Lake, 

Colorado. Quart. Jour. Geol. Soc, London, Vol. XLVI, 1890, pp. 423-428. 

. On some of the melaphyres and felsites of Caradoc. Quart. Jour. Geol. Soc, 

London, Vol. XLVII, 1891, pp. 534-544. 

. On a spherulitic and perlitic obsidian from Pilas, Mexico. Quart. Jour. 

Geol. Soc, Londou, Vol. XXVII, 1891, pp. 530-533. 

. On the sequence of perlitic and spherulitic structures : A rejoinder to a criti- 
cism. Quart. Jour. Geol. Soc, London, Vol. L, 1894, p. 10. 

Rutley, F., and Herman, D. On the microscopic character of some specimens 
of devitrified glass, with notes on certain analogous structures in rocks. Proc Royal 
Soc, London, No. 239, 1885, pp. 87-107. 

Sederholm, J. J. Studien iiber archadsche Eruptivgesteine aus dem siidwestlichen 
Finland. Tsch. Min. Mith., Vol. XII, 1891, pp. 98-141. 

Smith, G. O. The volcanic series of the Fox Islands, Maine. Johns Hopkins 
University Circulars No. 121, Oct., 1S95. 

De la Vallee-Poussin, Chas. Les anciennes rhyolites, dites eurites, de Grand- 
Manil. Bull. Acad. roy. Belgique, Vol. X, 1885, pp. 253-315. 

. Les eurites quartzeuses (rhyolites anciennes) de Nivelles et des environs. 

Bull. Acad. roy. des Sci. et des Lettr. et des Beaux-Arts de Belgique, Vol. XIII, 1887, 
pp. 498-535. 

Watts, W. W. Note on the occurrences of perlitic cracks in quartz. Quart. Jour. 
Geol. Sec, London, Vol. L., 1894, p. 367. 

. On perlitic structure. Geol. Mag., Dec. IV, Vol. Ill, 1896, pp. 15-20. 

Woods, Henry. The igneous rocks of the neighborhood of Biulth. Quart. Jour. 
Geol. Soc, London, Vol. L, 1894, p. 566. 

Vogel, Christoph. Die Quartzporphyre der Umgegend Von Gross-Umstadt. 
Abhandl. der grossherzlich hessichen geol. Landesanstalt zu Darmstadt, Vol. II, 
Part I, 1891, pp. 1-52. 

Vogelsang, H. Philos. d. Geologie, 1867, pp. 144, 153, 194. 

Wadsworth, M. E. Notes on the mineralogy and petrography of Boston and 
vicinity. Proc. Boston. Soc. Nat. Hist., Vol. XIX, 1877, p. 236. 

. On the classification of rocks. Bull. Mus. Comp. Zool. Havard Coll., Vol. 

V, No. 13, 1879, p. 282. 

Zirkel, Ferdinand. Mikroskopische Untersuchungen der glasigcn und halb- 
glasigen Gesteine. Zeitschr. Deutsch. geol. Gesell., Berlin, Vol. XIX, 1867, p. 784. 


Brogger, W. C. Dio Mineralien der Syenitpegmatitgiinge, etc. Zeitschr. fiir 
Kryst. u. Min., Vol. XVI, 1890, pp. 552-553. 

Cohen, E. Gottingsche gelehrten Anzeigen, 1886, p. 915. 

Cole, Grenville A. J On hollow spherulites and their occurrence in ancient 
British lavas. Quart. Jour. Geol. Soc, London, Vol. XLI, 1885, pp. 162-169. 

Cross, Whitman. On the occurrence of topaz and garnet in the lithophys.e of 
rhyolite. Am. Jour. Sci. (3), Vol. XXXI, 1886. p. 432. 

. The constitution and origin of spherulites in acid eruptive rocks. Bull. 

Philos. Soc, Washington, Vol. XI, 1891, pp. 411-444. 

Dana, E. S. Contributions to the petrography of the Sandwich Islands. Am. Jour. 
Sci. (3), Vol. XXXVII, 1889, p. 441-467. 

. Characteristics of volcanoes, 1891, p. 318. 

Delesse, E. Rechcrches sur les roches globuleuses. Mdm. de la Soc. ge"ol., France, 
Vol. IV, 1852, pp. 301-364. 

Hoist, N. O., and Eichstadt, F Klot Diorit Sliittmassa. Geol, Foreningens 
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Iddings, J. P. On the occurrence of fayalite in the lithophysse of obsidian and 
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. The nature and origin of lithophysse, and the lamination of acid rock. Am. 

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. Spherulitic crystallization. Bull. Philos. Soc, Washington, Vol. XI, 1891, 

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Iddings, J. P., and Peirfield, S. L. The occurrence of fayalite in the litho- 
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bascom.] LITERATURE. 9 1 

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Fig. a. — Quartzite. Specimen I. Slide 1 D. Between Waterloo and the Blue 
Mountain House. In polarized light, X 32. Illustrates the induration of a sand- 
stone by enlargment of the original grains. 

Fig. b. — Penetration Manebacher twin of anorthoclase in quartz-porphyry. Speci- 
men 23. Slide 23 D. From well, at depth of 40 feet near Clermont House. In 
polarized light, X 10 (about). 

Piedmontite tills the cavities in the crystal. 



a, Quartzite ; b, feldspar crystal. 


Bull. 130 7 97 


Fig. a. — Microperthic structure in a Carlsbad twin of anorthoclase in quartz-por- 
phyry. Specimen 181. Slide 181 D. Near the old viaduct on tlie Tapeworm Hail- 
road. In polarized light, X 85. 

Fig. b. — Broken feldspar crystal in quartz-porphyry. Specimen 166. Slide 166 D. 
Near Gum Spring on the Old Furnace road. In polarized light, X 28. The phenocryst 
has been broken and pulled apart and the cracks cemented with sericite scales. Tbe 
ground mass shows the micropoikilitic structure and the micropoikilitic areas are 
surrounded with sericite scales. 



o. Microperthitic structure in feldspar ; 6, stretched feldspar crystal. 




Fig. a. — Broken feldspar crystal in quartz-porphyry, Specimen 177, Slide 6b. 
Old Furnace road north of the junction with the Gladhill's road. In polarized 
light, X 115. 

Fig. b. — Quartz-porphyry. Specimen 25. Slide 64. Artesian well near Clermont 
House. In polarized light, X 28. 

It is attempted in this figure to show the continuity of orientation of a quartz 
phenocryst with the micropoikilitic area surrounding it, and to show (in a very 
crude way) the patchy effect given to the groundmass hy the micropoikilitic struc- 
ture. The quartz areas are bordered hy sericite scales. 



O, Quartz -a I bite mosaic filling crack in feldspar crystal ; b, micropoiki litic structure. 




Fig. a. — Flow structure iu an aporhyolite. Specimen and slide loaned by Prof. S. 
L. Powell, of Newbury, S. C. From tbe South Mountain. In ordinary light, X 24. 

Fig. b. — Chain Sjdierulites in an aporhyolite. Specimen 34. Slide 34 D. From the 
neighborhood of the Bigham copper miue. In ordinary light, X 24. Quartz pheno- 
cryst (now granulated) inclosed by chain spherulites. Clear spherules in center of 
dark bands are still preserved. 



a, Flow stiucture in an aporhyolite ; &, chain spherulites in an aporhyolite. 




Fig. a. — Augite-porphyrite. Specimen 69. Slide 69 D. Gettysburg Railroad, south- 
east of the Clermont House. In ordinary light, X 24. Ophitic structure outlined by 
iron oxide. Colorless areas are quartz, and the considerable irregular areas are 

Fig. b. — The same. In polarized light, X 24. The quartz breaks up into irregular 
areas, producing the mottled effect of the micropoikilitic structure. 



a, b. Augite-porphyrite in ordinary and in polarized light. 




Fig. a. — Perlitic parting in an aporhyolite. Specimen 279. Slide G. H. W. (un- 
numbered). Raccoon Creek, Franklin County. In ordinary light, X 88. 

Fig. b. — The same. In polarized light, X 225. A quartz-feldspar mosaic obscures 
all trace of glassy structures. 



b, Perlitic parting in an aporhyoiite in ordinary and in polarized light 




Fig. a. — Perlitic parting in an aporhyolite. Specimen G. H. W. Slide G. II. W. 
(unnumbered). Raccoon Creek, Franklin County. In ordinary light, X 40. 

Fig. b. — Axiolites in an aporhyolite. Specimen 77. Slide 77. South Mountain. 
In ordinary light, X 40. 




a, Perlitic parting in an aporhyolite ; b axiolltes in an aporhyolite 




Fig. a. — Altered spherulite in an aporhyolite. Specimen 280. Slide 280 D. Peach 
Orchard, southwest flank of Pine Mountain. In ordinary light, X 5. In polarized 
light the slide shows an even-grained quartz-feldspar mosaic, with the micropoiki- 
litic structure and with no trace of spherulitic crystallization. 

Fig. b. — An unaltered spherulite in an aporhyolite. Specimen 279. Slide G. H. W. 
(unnumbered). Raccoon Creek, Franklin County. In ordinary light, X 30. Ground- 
mass devitrified, but spherulitic crystallization still in a large part preserved. 








Fig. a.— Spherulitic aporhyolite. Specimen 226. Slide 226 D. South flank of 
mountain northeast of the junction of Copper Run and Toms Creek. In polarized 
light, X 28. 

The left-hand side of the figure shows the spherulites in ordinary light; the 
right-hand shows the same spherulites in polarized light. 

Fig. b. — Aporhyolite. Specimen 121. Slide 121 D. One-half mile heyond the 
Bigham copper mine, Old Furnace road. In polarized light, X 80. A crystal of feld- 
spar broken by the cleavage along the plane of former spherulitic crystallization. 
The figure shows the crystalline silica, which has replaced the spherulitic crystal- 
lization and which is much coarser in grain than that of the groundmass. 



a, Altered spherulites in ordinary and in polarized light; 6, chain spherulites with phenocryst. 


Bull. 136 8 113 



Specimen 153. 

Fig. a. — Ehyolitic structure iu an aporhyolite. Specimen 153. Slide 153. South 
Mountain. In ordinary light, X 120. 

Fig. &. — The same. In ordinary light, X 40. The structure is not satisfactorily 
shown in either figure. 



a, b, Rhyolitic structures in aporhyolites. 




Fig. a. — Rhyolitic structure in an aporhyolite. Specimen 153. Slide 153. South 
Mountain. In ordinary light, X 120. Aschen-structur of Miigge. 

Fig. 6. — Piedmontite in an aporhyolite. Specimen 162. Slide 482. Southeast 
flank of Pine Mountain. In ordinary light, X 120. 







Fig. a. — Amygdaloidal aporhyolite. Specimen 48. Slide 48 D. Bigham copper 
mine. In ordinary light, X 30. Epidote fills the center of the amygdnle and quartz 
surrounds the epidote. 

Fig. b. — Amygdaloidal aporhyolite. Specimen G. H. W. Slide G.H.W. (unnum- 
bered). Eaccoon Creek, Franklin County. In ordinary light, X 30. 







Fig. a. — Amygdaloidal aporliyolite with tridymite spherulites. Specimen 276. 
Slide G. H. W. Eaccoon Creek, Franklin County. In ordinary light, X 30. 

Fig. b. — The same. Specimen G. H. W. Slide G. H. W. (unnumbered). Raccoon 
Creek, Franklin County. In ordinary light, X 30. 







Fig. a. — Augite-porphyrite. Specimen 237. Slide 237 D. Head of Minie Branch. 
In ordinary light, X 120. 

Fig. b. — Melaphyre. Specimen 87. Slide 87 D. On Gettysburg Railroad, at fourth 
cut northeast of Monterey Station. In ordinary light, X 120. 




I N D E X . 

Allport, S., cited 37,57, 

Amygdaloidal structure in aporhyolites, 

characters of 

Analyses, chemical 33-34, 

Aporhyolites, character and distribution of. 42- 
Augite-porphyrites, character and distribu- 
tion of 69- 

Axiolitic structure in aporhyolites, charac- 
ters of 

Bailey, L. "W., cited 

Bayley.W. S., cited 09, 

Bell, Robert, cited 

Blandy, J. F., cited 17, 27, 

Bonney, T. G., cited 31, 64, 

Bornemann, T. G., cited 

Brewster, David, cited 

Brogger, W. C, cited 

Brongniart, cited 

Butler, G. W., cited 

Callaway, cited 

Cambrian rocks, description of 31- 

Chemical analyses 33 -34, 

Clark, John M., acknowledgments to 

Cole, G.A.J. , cited 

Copper ore, occurrence of 25 

Cross, Whitman, cited 38, 

acknowledgments to 

Dana, J. D., cited 37, 

Dana, E. H., cited 

Daniells, analysis by 

Daubree, A., cited 

Devitrification, proofs of 57- 

Diller, J. S., cited 47, 

Dolomieu, cited 

Ells, It. W., cited 

Evans, Lewis, cited 

Feldspar in quartz-porphyries, characters of 39- 

Feldspar in aporhj olites, characters of 44- 

Frazer, Persifor, cited 16-17, 24, 25, 26, 78, 

Fi itsch and Peiss, cited 

Futterer, Karl, cited 

Geiger, H. E., cited 

Genth, F. A., chemical analyses by. . . 34, 61, 62, 

Gerhard, cited 

vt il' 1 t-rsleeve, cited 

'Grant, U. S., acknowledgments to 


Groddeck, A. von, cited 

Hague, Arnold, cited 37, 

Harker, Alfred, cited 46, 

Haworth, E., cited. 47, 

Hayden, H. H., cited 24, 


• >•_> 


Page. ' 

Henderson, C. H., cited 25 

analyses by 61 

Hitchcock, cited 86 

Hobson, cited 68 

Hunt, T. Sterry, cited 17, 24, 25, 82, 83, 86 

Hidings, J. P., cited 31, 

3'< , 43, 47, 48, 51, 52, 55, 56, 61, 68 

acknowledgments to 50 

Irving, P. D., cited 31, 47, 55, 57, 60, 86 

Jackson, cited 86 

Jacks Mountain, geology of 21, 42 

Judd, J. W., cited 37, 68 

Kalkowsky, E., cited 59 

Keith, Arthur, cited 21, 22, 28, 29, 86 

Keyes, C. P., cited 32 

Kirwan, Pichard, cited 37 

Klockmann, F., cited 53, 59 

La Croix, A., cited 59 

Lang, H. Otto, cited 59 

La Vallee-Poussin v Ch. de, cited 55, 57, 60 

Lehman, A. E., superior topographic maps 

of South Mountain prepared by. . . . .18 

Lehman, J., cited 45, 64 

Lesley, J. P., cited 17-18, 21, 82 

Levy, Michel, cited 36 

Lieber, cited 86 

Lindgren, Waldemar, cited 47 

Link, cited 59 

Lithophysal structure in aporhyolites, char- 
acters of 55 

Loewmson-Lessing, F., cited 57 

Lossen, K. S., cited 59 

Ludwig, P., cited 59 

Mattbew, cited 86 

McCreath, A. S., analyses by 61 

Melapbyres, character and distribution of. . 69-78 

Mica of aporhyolites, characters of 46 

Micropegmatitic structure in aporhyolites, 

characters of 55 

Microx>oikilitic structure in aporhyolites, 

description of 47-51 

Milch, P. L., cited 45,64 

Monterey district named and denned 20 

ore deposits of 25-27 

Naumann, cited 36 

Nordenskjbld, Otto, cited 38, 47, 57, 60 

Osann, A., cited 59 

Perlitic structure in aporhyolites, charac- 
ters of 55 

Phillips, J. A., cited 31,68 

Piedmontiie in quartz-porphyries, character 

and distribution of 41-42 





Pinkerton, T., cited 36 

Pirsson, L. V., cited 47, 86 

Pumpelly, 31., cited 27 

Quartz in quartz-porphyries, character 

of 40-41, 45 

Quartz-porphyries, characters and distribu- 
tion of 39-42 

description of 39-42 

Raccoon Creek, description of amygdaloids 

from 56 

Reiser, cited 68 

Reusch, H. H., cited 08 

Reyer, E., cited 68 

Rhyolitic structure in aporhyolites, charac- 
ters of 54-55 

Rogers, H. D., cited. . . . 14-16, 21, 23, 25, 66, 71, 82, 83 

Rosenbusch, H., cited 35, 36, 37, 46, 57, 64, 68, 84 

Roth, Justus, cited 36 

Rutley, F., cited 40, 54 

Sauer, cited 59 

Schopf, J. IX, cited 14 

Sedimentary rocks, description of 21-22 

Sericite schist, localities of 64-66 

Shaler, 1ST. S., cited 86 

Slates, localities of 43, 64-66, 79-80 

Smith, G. O., volcanic rocks in Maine 

studied by 86 

Sorby, H. C, cited 31 

South Mountain, area of 13 

account of surveys in 14-19 

Spherulitic structure in aporhyolites, char- 
acter of 43-44, 51-54 

Suess, Eduard, cited 68 

Taxitic structure of rocks, description of. . . 57 

Teall, J. J. Harris, cited 37, 47, 68 

Tietze, cited 68 

Tornebohm, A. E., cited 31 

Trimble, Henry, analyses by 61 

Tyson, P. T., cited 10, 24, 25 

Van Hise,C.R., cited 31 

Vogel, C . , cited 59 

Vogelsang H., cited 59 

Wadsworth , M. E., cited 60, 86 

Walcott, CD., cited 21.22 

Wallerius, cited 36 

Williams, G. H., letter of transmittal by. . 11 

cited 13, 29, 41. 42. 47, 64 86 

Voung, A. A., cited 31 

Zirkel, F., cited 36, 49, 54 


& c